Farmville generally enables the higher processing power, so it will kill battery a bit.
Doing general web browsing with my mobile broadband USB stick in when I was stuck at an airport for a delayed flight, I squeezed 7:30hrs out of my 15" uMBP.
And it can vary from Mac to Mac, they do say up-to 7hours. 5Hours is generally the typical time you will get.
Put it this way : Mine is estimating 3:45hrs with 55% battery life. Thats at 1Bar Brightness, 3bars backlit keyboard, wireless on. It does vary and bounce around a lot. Don't worry about it.