How long will a sustaining focus hold a spell? If a shaman casts Increase Reflexes +3 into a sustaining focus, when will it stop augmenting her reflexes? When she tells it to? When she sleeps? Never? I'm tearing my hair out trying to figure out why she can't cast it at character creation and not worry about it any more (provided she never takes off the focus)

I'd also be interested in thoughts and house rules, in addition to canon definitions.

Sustaining foci go off when they're told to or they leave contact with their owner's skin. Or they get whacked on the astral (barrier, combat, etc).

There has been a lot of discussion of Increased Reflexes+3. In the modifications to Spellcasting that went into Third Edition, it is one of the few spells left where Force of the spell makes no difference in its effect.

Some rule that an active focus can be attacked from Astral Space. Some rule that can no longer happen after the magician becomes an Initiate and masks the active focus. Many GMs change part of the rules for this spell within their game so that it is more difficult to cast or requires higher Force. Here are links to prior discussions of the spell and what various GMs have done with it in their campaigns.

September thread
August Thread

If you want a strictly canon interpretation, active foci can be attacked from astral space (although "grounding" is out-- it would take me a while to explain what that was, so for now, just know that if someone mentions it, it's been removed in SR3); also, while a Masked foci can be attacked, you've got to detect it first.

Also, masking doesn't protect you from barriers and other indiscriminate astral problems like that. Just because it can't be seen doesn't mean it can't be stopped.

...or Fireballed....

Eh? You can't fireball on the astral plane, it being a physical spell and all.

I never said anything about the astral plane. What I meant was don't foci have to make an object resistance roll based on their Force against area-effect physical attacks? I seem to recall that somewhere. That would make a Force-1 sustaining focus pretty susceptible to just about any elemental manipulation (such as Fireball) and their relevant secondary effects.

He's talking about a fireball melting the focus into a lump of slag

There is a long discussion of carried equipment being damaged by spells in a prior discussion starting about here