I replaced my PSS seals last year at 11 years. I know the guys at PSS (they provided the first "points" awards for ActiveCaptain). I asked them a few years ago if the 6 year life was for real. They said that it usually lasts about 10 if you keep everything clean and check the rubber boot often for cracking. When I replaced mine, there still was no cracking but the boat was being hauled and I figured I was pushing my luck.
I had about 4,000 hours on them at replacement.