I IPL'd (booted) an OS/390 test/backup image about seven months ago.
It's not doing much of nuthin, but it's still sitting there running. No intervention at all, for seven months.
I love the big iron.
Yep, mainframes DO have their plusses....
I, however, have a firewall that hasn't been rebooted since it was installed two years ago. (HP-UX on a L-class, running Checkpoint FW-1). I did have to clear out some old logs a few months ago, before the filesystem choked.