Longest I've kept this sytem on is a month (P4 [yes, a P4 HT :( ]XP system from 05 or 06), give or take a week. Rebooting is a small hassle, involving stuff like irc server joining / identifying and the like, and getting various programs going again ect. Once the number of things needing a reboot get high enough (programs, antivirus scanning, updating) then I'll reboot. I don't have any kind of simple low power machine to keep running for handling irc, p2p, and the like, but once I do I can start powering off this system.
Ahh there is so much I'm planning to do once I get the money. Until then this rig is pleanty capable :)