With the default battle system I'm not sure (don't have any experience with TK3's default). I would try threads. See if you can get a thread to run that counts down a player's time left and kills them when it reaches 0. (Assuming threads work while the battle system is running.)
If you're working on a custom battle system, then of course there's plenty of ways to do it. I would imagine a simple counter that goes down as the player takes turns.
