ok just did a quick test and as I said up above if you key the rate as
Frame 29 = 100 (or whatever you have)
Frame 30 = 0

At frame 30 it will stop emmitting particles but the particles will still drift about for a while so to slove this follow the next part

Go into the particle shape node goto lifespan attributes and intally put you settings to:
Lifespan mode = random range
Lifespan = 1.
Lifespan random = 2.
General seed = 0.

If you don't like how fast they "die" then change the lifespan and the lifespan random

Hope it helps
Rob