I do remember a Freebsd 2.2 server, that after it's setup in 98, reached 800 days of uptime without any kernel upgrade, but the main reason for that, we were too afraid to do an upgrade, let alone a reboot, because if the server hadn't started, getting to it physically with a new boot CD, would have been a big issue. And that was the single server that the company had at that time, for a long time :)