Sorry didn’t reply earlier, but missed your post.
It sounds like you want the “stop frame” to be something other than 120 — just type the frame number you want into the last box in the playback toolbar, or ctrl-drag on the frame numbers near the 120 mark. That will move the line.
There are a couple of ways to “turn off” an emitter you don’t need. One is to use the “Active” graph. The other is to step the “Number” graph down to zero at the frame you want the emitter to turn off.
Alan.
http://www.wondertouch.com