most likely, you're using an application that ***** the power out.
i would check the activity app (in utilities folder)
and i would check with gfxcardstatus to check which video card is in use. If it is the AMD(ATI), then yeah it may suck more.
I have the top of line macbook pro 15" 2011 and do get over 7, sometimes, even displays 8hours or so if i am just browsing the internet.
It's been reported that software like Chrome, or Hyperdock would switch to the AMD/ATI card on their own.
You can overwrite that with gfxcardstatus.