How long can a Mac stay on without crashing?

[Syntho]

I always shut down my computers when not using them so I've never endurance tested a Mac before. If you're using an OS9 machine or prior strictly as an FTP server for example, how long do you think it'll stay booted before it decides to crash?

[nanopico]

I ran an iBook for 6 months without shutting down just putting it to sleep and never rebooting. I only had to because the battery died when it was not plugged in.
I ran an old Performa as web server test without issue or needing a reboot for almost a year. Power outage caused that one to reboot.

A co-worker used to work for a newspaper in their web services area back in the early 2000's.  They ran their DNS server of a 9600 with 8.6. He said it ran without reboot the entire 4 years he was there and for several years afterwards (as he still kept in touch with the people there) without ever being rebooted.

I would say though it depends on what all your doing.