AT my Uncles company they have an APC Online one which they got for Second Hand . Although the price is a bit high the UPS does give them at least 1 Hour of support on their system.
As far as the above UPS are concerned the maximum time you might get is 10 minutes and thats after being optimistic. My system with a 600 VA Ups gives me probably 3 minutes max. I've so far had a good experience with APC and V-Guard .