My A3000UX has been running nonstop for a year and a half. Before that, swap motherboards with my plain jane A3000 and that ran for two years. Before that, back to the A3000UX which ran for 7 years nonstop aside from an hour here and there changing the hardware configuration. I have not had luck running C64's for too long. I blame bad power though.
One thing I recommend: Get a GOOD UPS. I have a bunch of APC ones that have failed and put out *lethal* power. I honestly don't know what's better though.