Mine are running all of the time, including during power outages, and are only shut down for physical maintenance and reboot for software maintenance.

This is a little variable through. Windows hosts tend to require more frequent software reboots in my experience. About once a year, I physically open each device and inspect, clean dust (fairly rare to find it for my setup though), and perform upgrades, replace old storage devices and such. Otherwise I leave them alone.

I usually get about 5-7 years out of the servers and 10 out of networking hardware, but sometimes a total failure occurs unexpectedly still and I just deal with it as needed.