I know you can use IPO curves to make the tail of say a long stream of particles fade away, but what about making all the particles fade at once?
eg. trying to make a rocket launcher fire, the rocket fires then leaves a trail of smoke that I would like to all fade away after 50 frames at once.
Anyone know how I would do this?
Cheers 
Hi,
Dont know if this helps, press i to insert key with mouse over halo size, you can then key down to zero. This may look ok, depending on your particles.
Any material settings you key on a particle emitte will be mapped to the lifetime of the particles emited. In order to change this behavior you need to parent a mesh to you emitter and set the duplivert button in the emitters object buttons. If you create a mesh with only one vert and set its mat to halo it will look the same as the normal particles when dupliverted, but can have absolute keyframes set