should last as long as the clutch and should be replaced with a new clutch install..what kinda cable are you using and do you have an alum. quadrant and/or firewall adjuster alot of times improper adjustment of the cable causes premature throwout bearing failure.
Many things could make a T/O bearing wear out too fast
1. Bad clutch cable
2. Bad bearing retainer..aluminum is stock they get a groove worn in them
3. Bad pivot ball on bell housing
4. Bad clutch fork
5. Clutch not fully engaging/disengaging.
6. Faulty bearing from the start.
I have had steel bearings that have been fine. I had a plastic bearing from Centerforce that I was scared to use, but it was fine too.