Assmebly language & PIC - time duration

Oblio

Hey everybody,

I have a program in assembly I've been making to determine the frequency (period) of an input analog wave.
It is starting to work, but I need to specify the PIC or program to only run for a specific amount of time, NOT a 'until x-amount of points are taken'. Can anyone tell me, generally how to program it in so that it starts and stops so many (micro...whatever) seconds later?

thanks!
-adam