I have 3 that are on 24/7. 2 of them run at 100% CPU running ASIC miners, it seems I only have a problem about once every month or so, typically they reboot them selves.
1 is a media machine that has been playing a customized media loop for nearly a year, I get tired of it and update the firmware everyone in a while to see if I can break anything but no problems it just goes.
Another is used here and there when I am trying out new projects, I am about to stick it in a fake security cam housing and attach the real cam to it. I only point this one out because it has been dropped and set outside and been in the sun, the cold, and everything else you can think of but water. No problems.
I also have 6 more that have not seen their intended use yet.