I had a windows sever that got 329 days uptime before I rebooted it (a reboot it didn't even need). It was running fine and had no issues. Shows how long it can be left on.
Oh, and this server has never been updated, it runs with its firewall off, and its never been hacked.

Obviously I'm not going to tell you the address of it!
My production servers usually get a reboot every 2 weeks for some reason or another. The longest any of my production servers have been online for is 60 days before a reboot.