And I would say it would be a max of 25 hours, and a minnimum of 10 hours. Resistence + age will also decrease the time.
You can check it out by downloading the software and installing your UPS via USB cable and disconnecting your power cable while your 20 watt device is connected. IT should be able to tell you how much time is left just using the battery.
Volt Amps and Watts are both a determination of instantaneous power, a 650 W UPS does not necessarily have 650 Watt hours of power stored in it, it may only have 200 Watt hours of power stored in it but can deliver up to 650 Watts at peak draw, it wont last very long but it can support a bigger item temporarily.
What UPS do you have? Look at the up time it promises for given loads, that will help you determine what it would be for such a small load.