A good sized ups (good enough to keep a couple pcs and monitors on for 10 minutes) would be good for a normal power outtage.
That said, obviously it's variable!
I have an old school (hah 4 years old
) APC 720 watt ups attached to a 'fallout' power strip in my setup.
The strip powers only the heater and a single maxijet1200.
(I have the whole setup through a controller, but it's all staged properly for a power outtage)
I got impatient and turned the power back on after ~30min of waiting for the ups to die.
For my area (metro atlanta) the power will be back on within a couple hours, so 30 min is enough.
Newer model ups units offer a much better bang for the buck in terms of power output, so with the modern equivalent of the unit I have, I'd imagine a couple hrs of output maybe...MAYBE... (obviously, do the math right..)
That said, if your area is prone to brown/black outs that last a long time, a gas generator makes alot of sense if you are dumping 5k into a tank hehe.