Depending on the power rating of your PC, and the amount of power your UPS can provide converts directly into the backup it might provide. A generic UPS being used to power an equally generic PC might last for upwards of 20-30 minutes of run-time at full battery charge. Stronger UPS are also available on the market that can let you use your PC for another hour before their battery eventually runs out. More powerful PC's almost certainly would need better and bigger UPS for them to run. A UPS' capacity is measured in VA (Volt-Ampere) and gives you a basic understanding of how much power it can generate for its connected components.