Final Cut and quicktime both chew up the battery to some degree, especially FCP.
I can comfortably get 7 hours plus out of my mid 2010 15" i5 on light useage (web browsing etc) but if I start hitting the meaty apps then that battery will plummet.
Ripping a DVD in handbrake will reduce it from 7 hours plus to around 90 mins for example.
Also, do follow GGJ`s guide and calibrate your battery, it won`t improve the life of the battery but you will get more accurate estimates.
Better yet, switch to percentage instead of time remaining, you stress about it less I find...