UPSs are usually advertised by their VA rating: 650VA, 1500VA, 3000VA, etc. The VA stands for VoltAmps. VoltAmps are the amount of power you'd get out of a basic power equation if there weren't any losses or inefficiencies in the system. In a perfect system, VoltAmps would equal watts because there wouldn't be any losses. But typically there is about a 30% loss in efficiency (called the power factor). Long story short, if you take the VA rating and multiply by .7 you'll get a good idea of the amount of power (watts) a UPS can support. So a 1500VA UPS can support a number of devices that have a power draw of ~1050 watts. (More expensive units are usually more efficient, read the fine print).
Find out what the power draw is for each of your devices. Some manufacturer's will list it in actual watts, for others you'll have to multiply the voltage (typically 110 in a U.S. home) by the number of amps it draws. For instance, my 21 inch CRT monitor draws 1.4 amps at 110 volts, for a power draw of 154 watts per hour. Add up the power draw for your devices and then pick a UPS that can supply that many watts.
None of the above has anything to do with *how long* your devices will run on a UPS. That's a function of the number of batteries in the UPS, their Amp-Hour rating, how they are wired and the amount of energy your stuff is pulling out of them. Many UPS companies will have charts listing the run times of their units at 1/2 and full draw. Find a unit that has the necessary VA rating (or higher) and the run-time you need.
Here's a runtime chart from American Power Conversion (APC) listing a bunch of their units: http://www.apc.com/products/runtime_...?upsfamily=165
If you want to draw 600 watts for an hour (5.5 amps @ 110 volts), you need something like a 3000VA UPS (~$1200).