I had bought a UPS for this hurricane season. Turned out I didn't need it, thankfully, but hey, peace-of-mind for future power outages. Mine is rated at 450 Watts. Cost quite a bit too, a little less than a PS2. Anyhow, I'm not sure about the duration. The only thing I have connected to it are 2 AquaClear filters.
One AquaClear is rated at 6W. Electricity is kinda weird. Watt hours, this and that, I have no idea. But I believe you will get a pretty close estimate if you just divide 450 by 6. So for one filter, it would run for 75 hours. 3 days, 3 hours. Then again, I am not positive. I'll have to ask this guy I know that is an electrician to be sure.