Depends on your bio load, the temperature, I try and make sure my filter is off for no longer than 2 hours. After that, the filter starts to die. Filters take relatively little power, its the heater that would suck a UPS down quickly.
Problem is, the inverter in all UPS's themselves are relatively inefficient, draining the battery faster than the filter running does. You'll get 2-3 hours MAX from any consumer UPS even with a tiny load on it. So, If I think the Blackout will be a long one, I generally shut off my UPS manually, and turn it on once an hour for 5-10 minutes.
My last test of this was when I had my electrical panel replaced. my APC 550va UPS lasted about 2.5 hours continuous, but when the hydro guy never showed up to connect my power back on, I turned it off. I turned it back on 3 or 4 times over the next two hours for a runtime of 3 hours. I was impressed.
When I had a 100 watt heater plugged into it for my move, it lasted 40 minutes.
"In this hobby - Bad things happen over night... The good things take time."
In Progress: 75 Gallon Planted Discus tank
Baby Sitting: 10 Gal Tank with single Convict