I gotten a Windows 95 machine up to the 49.7 day limit. The key was the machine was hooked up to some special scanner that didn't get used that much, so the computer spent 99% of its time idling at the desktop. Once I realized it was getting close I figured out when exactly it was going to hit the limit so I could witness what would happen. Which turned out to be nothing, until I clicked the mouse and it BSOD'd.
I've also managed to get a Vista machine up to the 497 day limit. In that case the computer was still running okay other than the networking being hosed.