I assume you want to change the 'lifetime' of the particles?
If so click on your emitter and the control you want is in EMITTER / NUMBER. reducing this number over time will cause the particles to die out.
To check the keyframes go to your TIMELINE
Click on the emmitter you want to change and look for 'NUMBER' in here you can change the keyframes and extend or reduce the 'lifetime of the particles'
hope this helps