If it wasn't for power outages, 3 systems would be left on 24/7. Alas, a stupidly sensitive electric supply defeats this. A further 3 systems remain in S3 Sleep mode when not in use.
As it stands though, my PowerMac G5 (used as a Server) has been up and running for 9 days. My record was about 4 months before I had to work on something and reboot it.