Stupid rule question:

When I use my next action to go on full defense, when does it end? When I should have taken the action, or when I get another action (so 1 action later)?

How many times can you use an Interrupt Action in a Combat Turn? Could you potentially "borrow" infinite actions this way?

When a character uses an interrupt action, such as Full Defense or Riposte, he uses up his next available action. Characters may even take an interrupt action when they no longer possess an action that pass or Combat Turn, by “buying ahead” their very first action of the next Combat Turn instead, but only the first. They cannot borrow indefinitely (Interrupt Actions, p.148, SR4A).

Since your initiative comes first, you should be able to keep doing so, but at a -1 dice pool modifier for each additional consecutive defense.

This came up in one of my friday night fights. Basically, you spend your next available complex action to dodge. So,

Initiative: You 10, 2 passes Goons 5, 1 pass.
On 10, you shoot a goon.
On 5, a good shoots you. You full dodge.
On 10 IP 2, you're dodging the attack from the previous round, using your complex action. You still have a free action.

This means having more actions than your opponents is very useful, less you get stuck in a 'dodge loop', i.e.
On 10, you attack.
On 5, a goon shoots you.
On 10 (the next IP), you're dodging.
On 5 (the next IP, you get shot again, and you dodge again.

and so on, and so forth.

Great...now I have the Simpson's "Dr Zaius" to the tune of Amadeous stuck in my head...

The other side of this coin, when does the Full Defense end?

Say I have two characters, Bob with 2 IP and Joe with 3 IP. Bob gets 5 hits and Joe gets 2.

IP1:
Joe: Joe fires at Bob. Bob interrupts with Full Defense.
Bob: Bob must use this action for Full Defense. He uses his Free Action still available to radio for back-up.

IP 2:
Joe: Joe fires at Bob again. Question 1 Does Bob's previous Full Defense action apply here?
Bob: Bob fires at Joe. Joe interrupts by taking Full Defense, sacrificing his first IP next round.

IP 3:
Bob: Bob fires at Joe. Again, would Joe's previous Full Defense apply here?

ROUND TWO: Joe rolls 1 hit and Bob gets 3.

IP 1:
Bob: Bob fires at Joe. Question 2 If the Full Defense applied in the last IP of the last combat round, does it still apply?
Joe: Must use his action for Full Defense.

God I hope that made sense...closest example I could think of.

Sorry for cutting out the example, but I wasn't sure what the hits were...but onward.

Full Defense ends on the next IP you get that wasn't spend on the Interrupt.

So let's say you have a 10 Init/3 IP vs. 3 goons with 9 Init/3 IPs

IP 1,Init 10 you shoot and kill goon 1
IP 1 Init 9, goon 2 shoots and you Interrupt for Full Defense. Your second IP is gone, you roll full dp for defense.
IP 1 Init 9 goon 3 swings melee, you roll appropriate dp -1 for second defense. NOTE: If your character had some ability to allow another Interrupt action, like the "Throw" Martial Art manuever, you can NOT interrupt further into the future.

IP 2 Init 10...aborted
IP 2 Init 9 goon 2 shoots, you are still in Full Defense, so now it is dp -2
IP 3 Init 9 goon 3 swings, you are still in Full Defense, so now it is dp -3

IP 3 Init 10 you shoot goon 2...and so on.

Bob's full defense works until he gets another action. Of course, he takes a -1 for having defended from a previous attack. That's what I was referring to with the 'dodge loop'.

Also, sorry about the song I hate every Ape I see, from Chimpan-A to Chimpan-Z...

So the answer to both questions is "Yes" then?

Not worrying about the -1 as that's pretty self-explanatory...just trying to figure out the timing...

Yes, as far as I'm aware if you go full defense it sticks till;

A- you choose to use your simple/complex action for something else.
B-your full defense is negated by the accumulation of -1 to consecutive defenses penalty (why bother with full defense at that point).
C- you have finished that particular combat encounter.

How do you mean? You'e used your next available complex action to go full defense.

So,
Bob goes on 10, Guards go on 9
Bob shoots a guard on 10.
On 9, the Guards shoot bob, and he goes on full defense.
Next pass, on 10, Bob is 'full dodging' with his action. That full dodge is now over, and he can take a free action.
The guards shoot bob again, and he goes on full defense again, or chooses not to. Let's say he does.
The pass after that, on 10, Bob is 'full dodging again', and can take a free action.

etc etc etc

Throw is used as part of a Full Defense, so it is part of the Full Defense Interrupt Action. A scenario:

Huckabilly Throwya is a Troll PhysMage Martial Artist with the Throw Manuver (holy capitalized proper nouns, batman!). He has a dicepool of something fairly big, and is partaking in an ancient Martial Artist tradition: Standing in the middle on a precariously thin bridge, defending against a million, zillion ninjas attacking from both sides. After spending his own Complex Action on brutally tearing the arms off of one ninja to freak the rest out, the million, zillion ninjas attack. When the first ninja attacks, Huckabilly Throwya falls into Full Defense, wins, and Throws the useless ninja into the gorge as if his favorite color weren't blue.

Well, Huckabilly Throwya has two Initiative Passes for the Combat Turn, while the million, zillion ninjas have one Initiative Pass. The million, zillion ninjas are also all under the effects of the Movement power, and thus can send wave after wave of wimpy ninjas into Huckabilly's waiting arms. Since every one of the million, zillion ninjas have one IP, they are all dying inside of Huckabilly Throwya's single Full Defense.

Lets say that the million, zillion ninjas throw themselves into the greedy clutches of Huckabilly Throwya, and have reduced his Full Defense pool down to 1 die. Then on Initiative Pass 2, the Complex Action that was interrupted to comes up and goes by. One Reaction count later, the big bad boss comes out to play, laughing at his hapless old master, now helpless in his big bad clutches. The big bad boss charges in to attack Huckabilly Throwya, who calls a Point of Order.

"Look, big bad boss who used to be my best student, didn't you learn anything from me? Read the rules for "Defender Has Defended Against Previous Attacks" it reads: "If a character has defended against at least one other attack (melee or ranged) since his last action, apply a –1 cumulative modifier for each additional defense roll."! Don't you see? my action came by and now you're screwed!" His student laughs heartily, and responds with:

"No you dumb old bastard! The Complex Action time period came by, not your action! You already blew that by Interrupting to Full Defense!" and starts laughing his evil boss laughter, which is suddenly cut off by his screams of "Hooooooooooooooow?!?!" as he falls into the Abyss with his mooks.

"Because, dumbass," Huckabilly Throwya explains, "even if you are right and I'm wrong, you spent too many free actions monologuing and my turn came again! See you in hell!"

And that answers that question.

A couple of points here Simon:

1) You read that as saying each of the million uses of the Throw manuever is allowed for one interrupt action? I'm not sure I see it that way.

2) In your example, the Thrower goes in an IP, then interrupts his next action for Full Defense. That is fine, but when the Thrower then attempts to Throw as part of that Full Defense, he doesn't have an action to "abort" to. The rules and FAQ pretty clearly state youcan not borrow indefinitely into the future.

Thrower has 3 IPs this turn.

IP 1 he attacks and then is attacked, interrupts, sacrifices IP 2 to go Full Defense. He can NOT interrupt to Throw because that would be sacrificing IP 3...sacrificing into the future.

Now how it could work is this way:

Thrower has 3 IPs this turn.

IP 1 he delays (or is attacked before his phase) so he goes Full Defense by interrupting and sacrificing IP 1. The Thrower succeeds at defendingamd chooses to interrupt again to Throw the opponent by sacrificing IP 2 which he has available. He can not throw a second opponent on IP 1 because he has noavailable action to sacrifice.

You are correct, I was misreading the Full Defense rules. I guess the big bad boss who used to be his student totally kills the hell out of Huckabilly Throwya with ninja mooks.

Putting it as simply as possible, an Interrupt Full Dodge action counts until the next time your spot should have come up in the rotation.

It's possible for this period of time to be extended only by your Initiative dropping due to Damage Modifiers.

So, if the order is:

12: goon 1
10: you
08: goon 2

Then a dodge cycle goes:
IP1
12: goon 1 shoots at you. You use an interrupt to go on full dodge.
10: your full dodge action ends, you use a free action
08: goon 2 shoots at you. You use an interrupt to go on full dodge.
IP2
12: goon 1 shoots at you. You are still on full dodge.
10: your full dodge action ends, you use a free action
08: goon 2 shoots at you. You use an interrupt to go on full dodge.

CT2 begins - Roll Initiative

Gah...I am away from the rules right now...

Are you sure on IP 1 Init 10 the character's full dodge ends? That is the action phase the character sacrificed to interrupt FD.

I was thinking the Full Dodge doesn't end until the character gets an action phase.

Full defense last until the character's next action phase. When used as an interrupt, the action phase that the action was borrowed from is the character's next action phase.

Borrowing an action from a future action phase doesn't eliminate that action phase, just the complex action in that action phase.