The rules for fastcasting are pretty clear.
Fastcasting is a response to something, always have a chance to respond. The ease factor, in combat, is Quickness +Finesse + Stress Die against the initiative point. Each spell after the first takes a cumulative -6 for fast casting.
We'll take Marcus, since I designed him to be a fastcast son of a gun.
His Quickness is 4, Finesse is 3 (Ignem specialty), and we'll presume an average die roll of 5, for a total of 12 for his fast cast speed. That means, he can fast cast defend against someone at initiative point 12. Let's say someone also acts against him, but at initiative point 6, he takes the -6 penalty and he can still respond. He could also respond to someone at initiative point 0 or lower. Now, this is presuming that all the stress die rolls are 5 or greater. He can keep fastcasting until he fails the speed roll.
Each fast cast spell must be either spontaneous (with firm voice and bold gestures, so no extra bonuses), or mastered for fast casting. Fast casting is a "spontaneous" reaction to some incoming threat.
Marcus can reasonably respond to two normal magi (they will typically be 5 or lower, given most magi don't put charactersitic points into quickness).
Marcus is also effective as a first strike vehicle. His initiative starts at 7, (Quickness 4 + Fast Caster 3) On average, he's acting at initiative point 12. If he's launching PoF, it's at 14, because he has Quick Casting x2. In the above example, presuming that Marcus is responding with PoFs (imagine this is what happened at Thera) he can cast 18[1] PoFs in a round at initiative points 14, again at 8, and again at 2, on average. Marcus is kinda my munchkin player. I wanted to see how over the top he could be. He was purposely designed to beat the Flambeau at one of their contests. However, he could only do it in Normandy, since to compete in the Dimicatio at the Flambeau Tournament, you have to place well in the Certamen tournament (not sure I see the wisdom of that as they measure vastly different skills/Arts, but something for another discussion).
[1]Since initiative points can be negative, it is conceivable that he could act again at initiative point -2, but...