I sleep my G5 when I'm not using it. I had an uptime of nearly a month once, but always some update forces a restart. My mom does the same thing with her G4 iMac. She used to use a Power Mac 6500 and that thing could go for months without a reboot.

The other computers I use sparingly so they are turned off. In my new apartment my roommates and I are going to set up a Mac (probably a Beige G3 or something) as a file server so it will be on 24/7.