On maximum performance the battery won't last long at all.
Some of the frankensteiner laptops get as little at 12 minutes from their battery, if your battery more more than 9-12 months old, its probably losing capacity.
All my Dell Batteries start to slow degradation after around 9-12 months with many giving less than 30 minutes after 18 months, and being virtually dead after 24 hours. The battery I bought late 2009 currently gives me less than 15 minutes.
Try and see if it works fine on mains power, if it does its likely a battery or settings issue.