The discription on the heal spell says that drain is DV-2. Is that the amount of damage the vitcem has before the heal spell or the amount that is healed?

Before.

You take drain immediately and have to hold the spell for several minutes before the damage is actually healed.

Correct, so if your teammate has 8 boxes of damage and you want to heal him, the base drain is 8 - 2 = 6 DV. Draco is also correct, you need to Sustain the spell for the number of combat turns equal to twice the drain value. That would be 12 Combat Turns in this case. Yes, Heal is not practical to be done in the midst of combat, that is the intention.

I should also point out that the "sustain for drain value x 2 turns" rule is for any spell with a duration of P (Permanent). I get the impression that many people have missed this vitally important tidbit.

Examples of these Permanent Spells:
Antidote
Cure Disease
Detox
Heal
Influence

I just want to clarify what Draco18s wrote, the damage vanishes immediately after the spell is cast and the subject gets all the benefits of having less damage than before. However if the mage loses his concentraction during the next DV*2 rounds that effect is gone and there cannot be another attempt to heal this wound.

If you use the FAQ you have to maintain the range condition untill the spell becomes permanent.

You just have to make contact with the subject, right? You don't actually have to touch the affected area, right? I mean, it is one thing to put pressure on a bullet wound, but I'm not sure I want street people to know I have Cure Disease.

Wait wait... what?

Unless I've missed an errata or something, the core book says that a character can be healed once magically per set of wounds. I've checked the SR4A Changes document, the FAQ and the errata for this clarification you mention, but it doesn't say that anywhere. It doesn't say they can have only 1 attempt to be healed magically per set of wounds. If the character is disrupted, counterspelled, or whatever before the effect takes, then it's a failed attempt, and as such the wounded character still has 1 available magical healing.

I always ruled that the heal spell counts as soon as it is cast not when its effect becomes permanent.
Since the description of the Heal Spell does not specify otherwise, the damage boxes are removed as soon as the spell is cast, and the subject thus is healed.

If the target were only considered healed after the spell is permanent, a skilled mage could always release the spell before permanency to get a more desirable result (i.e. more hits) as long as he can soak the drain. I don't think that is RAI or RAW.

I think you have something mixed up. The drain is not based on how much damage that was healed the drain is based on how damage the subject is in the first place.

Example:

Critical Carl has taken 8 boxes of damage and is bleeding badly.
Merry Magician runs up to Carl and casts a Heal spell. She rolls 5 hits.
not-so-Critical Carl is now at 3 boxes of damage (8 original - 5 hits = 3 boxes).
Merry Magician must now resist a drain of 6 DV (Carls 8 boxes of damage - 2).

The spell must be maintained for the entire duration of 12 Combat Turns to have any effect at all. That's how I've been playing the spell.

Nope, that's my understanding as well.
Here we differ. According to page 173 f. of SR4 and page 183 f. of SR4A Step 5 of the spellcasting sequence is the determination of the effect and drain resistance and the ongoing effects are steps 6 and 7. So the effect is there before it is permanent.

Also Only says that the spell must be sustained for the effect to become permanent, not to manifest at all.

This is my reading on it as well. However, this discussion has given me an interesting idea. Although there might not be a large call for this, it struck me as a viable tactic for those rare occasions.

Situation: Part of the job is a zero kill count. Conflict with a primer-runner type opposition ends with the runner dropped, but not dead. Knowing the runner has allies your group mage can heal the enemy then drop the spell before it becomes permanent so that he had been healed but is no longer able to accept magical healing.

As I said, situation limited, but still a fun way to use a beneficial spell to hinder an enemy.