How much back up dos your computer UPS give you?

I had two UPS for the same computer and both gives a 45min back up to my pc. Normally all the UPS give a minimum of 20 min backup. If you are getting lower than the specified try to read manual about UPS and follow the instructions that are specified. Normally any UPs works best when they fully charged and they completely drained once a week. If you facing low battery backup., try to find the problems. Are you loading over the specified. Some monitors need high wattage. Do battery cleanups regularly to get more backups.

The backup time varies from ups to ups even though they are manufactured by the same company....My UPS gives a backup time of abt 15 min

Not sure, but I think ours is 20 mins. I don't have one at home just at work.