am using relays to control high power motors but need to make very fast on off pulses (20ms bursts).
I don't think that mechanical relays that are able to switch that fast at all. The usual contactor for a 4KW motor (which would I regard as the smallest size for high power) needs about 100ms to switch the motor current on and off.
What you should be looking for is a SolidState Relay. They have no mechanical parts.
How long do they last?...it's meant to be installed for 6 months, so will be used everyday
I would place my bet at 2 weeks 
BTW:
What the purpose of switching the motor current that fast ? There might be better solutions around ...
Eberhard