For VMSClusters 8 to 10 years seems to be a pretty common uptime for long lived clusters. I worked a gig a few years ago that had a VMSCluster that was up for 19 years at that point. Started on alpha, migrated to itanium, then I helped migrate it to x86. It was the oldest I've personally seen, though I have heard legends of much older.

My current contract has a PDP-11/70 running RSX-11 that's been up since 1992. It's uptime is longer than mine! Poor thing is getting retired next year though.