last year after 500 passes on my Crower HIPPO severe duty lifters, I thought they all rolled good but decided to send them in just to make sure. Crower rebuilt 7 pairs for 50.00 bucks per pair and found 1 pair not rebuildable and charged 112.00 for that new pair. This is with .550 lift, 550lbs open pressure and turning about 7400 through the traps. I think how aggressive the cam is and rpm's would help determine how long to go between rebuilds. Jim