hmm ok, math isn't my best subject, but I'm going to assume that my computer uses maybe 20 watts in sleep mode. So if my UPS has a 600W capacity, it would be
1) The TIME it lasts is stated at the manufacturer or store site.
2) Be WARNED that many models don't work. If you have a PFC circuit (any "80 plus" model must) in the power supply then you need a UPS that supports PFC (adequate sine wave).