You must be doing some very intensive things with your Pebble. I have the automatic backlight turned on (turn on when I shake it), and mine regularly lasts 3-5 days, even when I go running regularly, which happens to trip the automatic backlight for most of the run.

I also use it to track yards I go swimming, Runkeeper for tracking my runs, and miscellaneous things like the music app and the Swarm app.