The longest uptime I had was
812.5 days. This was a FreeBSD 4.11 system. Probably would still be going if it wasn't for external factors (had long duration, scheduled power outage). That box has now been going for 264 days without reboot (after the scheduled shutdown, the OS was upgraded to FreeBSD 6.3 and the hardware was updated a bit).
So 3 years with one scheduled reboot on generic white box hardware. Not too shabby.
