depends on your pc config.to get an idea do this:
1.go to extremevision online power calculator eXtreme Power Supply Calculator
2.calculate your pc load(select system load as per your situation when watching movies/browsing etc as default value is 90%).
3.calculate total load on ups by adding load of pc calculated in step 2 & of other devices connected to ups like monitor.
4.backup time=(58320/total load calculated in step 3) seconds.