I presume you're referring to Apple's BT mouse.
My wife has one on her iMac G5, and we get a good month out of the batteries. It all depends on how much you use it during the day. Using it for 8 hrs per day solid, and the batteries will drain no matter what.
However, if you think you batteries are running down prematurely, consider changing the mouse pad - its possible (a wild theory) that the pattern of your mousepad is preventing the mouse from going to "sleep" when it senses no movement after a while. Perhaps the pattern is somehow keeping the mouse busy, continuously oscillating between two focal points (hey, I said it was a wild idea). But you never know till you try, I guess.