Recently my cousin bought a Moser Baer 600VA UPS for. The official link says it gives 10-15 mins backup with a 15" CRT. This time should be,more with an LCD, as they consume considerably less power I know you have a better budget & you may want to go
in for a better brand or maybe a higher capacity UPS. I gave you the above-info just in case. I personally use Numeric UPS. It has got a serial port at the back using which the battery status, voltage fluctuations, etc can be monitored in real time using the bundled s/w. However it is really old (around 5 yrs) but works like a charm even today.