Just do it like in fear.
20 seconds of battery life,
5 Seconds of recharge time.
In dark areas, it's be a great way for traitors to sneak off a knife kill without anyone knowing, and would make ZS a hell of a lot more fun when you're in a dark corridor, hear zombies right behind you, and your flashlight goes out.
//Edit
Someone can do the lua, I doubt it'd be that difficult to program the flashlight to automatically turn off after a certain period. I just think it's the recharge time that would be the hard part.
Someone else can make the derma for the flashlight,
Cutout from in-game SS :L
If we want to go all out, I can actually rip the flashlight sounds from the game itself. c: