The absolute start and end of a particle emitter is set in the timeline - to have particle start emitting at a certain frame, all you have to do is go to the Timeline tab, select Overview mode, and drag the timebar for the particle emitter to the frame you want the emitter to start generating particles. Combustion also uses the current frame as the start frame whenever you add a new emitter to the workspace (i.e. if you're at frame 15 when you click to add a new emitter, that will be the start of the particles).
Other methods, such as animating the number of particles, will also work, but they'll require more work afterwards if you need to adjust the timing, since you'll have to move keyframes (and related keys if you've animated other properties) whereas this method is basic drag and drop.
-- Greg