Agreed with Daromer. However if the UPS is a server grade unit, then they are designed to run basically hours on end w/o let up.
Cheap house units are like he says, designed to only run about 30mins or less as they will heat up a LOT. I found this out when my power was out a few years ago for about 2 weeks. I had a small 500VA APC dumb unit. It was connected to a 12V Lead Acid car battery. It ran pretty good. However, the board warped a little, and discolored at some of the FETs and the poor cheap digital voltmeter I had laying on top there was a hole melted through the case where it was sitting on top of the FETs. Fortunately nothing actually stopped functioning. But I don't run it on long loads like that now. It only serves to keep my server/router from getting knocked out when our power blips. It's connected to a lawn mower battery now, though