The rated time is 10-15 minutes according to the spec, but I have never had the need to test that limit. Here's is what I have running on the 650VA UPS outlets:
350watt tower case running PIII and three video cards; 21" monitor 19" monitor (17" monitor not on the same backup) DSL modem Cable modem
Out here in the central New England hills, thunderstorm often discrupts power supply, then it comes back within minutes. Sometimes it comes back within seconds, and that killed the hard drive on my back-up computer. So now that box is also on the UPS, but usually not running during the day.