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:
I like this Idea, It adds a little bit of fear, and realism into the game, Plus how hilarious would it be to run up behind somebody as a zombie and hear them freak out It'd be like playing Amnesia: The Dark Decent, Alllll over again, This adds a bit of spiceh to my mild of video games,
Plus, If the flash lights go out and the traitor knifes somebody, Nobody would know who did it. (How to bring a balance to this? It's simple, The players just have to think strategically, Have one or two flash lights (Depending on the group size) on at a time, The Traitor can't see the other two, But if somebody dies, You'll know who did it, Just make it so that it gets darker when the flashlights are turned on so you can't see the players that don't have one on, Spice it up a bit and give people a scare, Some fear to get that adrenaline pumping and get into the game.
EDIT: Also, Mayyyybe decrease the recharge time.