I do the same thing. I can't remember for sure, but I think the batteries lasted two and a half weeks or so of almost constant use at home and at school. Figure about 6 hours/day maybe more. Of course, they're just AA's so you can get them anywhere or just keep an extra pair in your bag. When I got the low battery warning I was still using the mouse 3 hours later!
Also, if you're a programmer looking to mess around with the BT api in Windows XP SP1 be careful what adaptor you get. Most adaptors out there use the WIDCOMM BT stack and API which is not available for free for developers. I have a belkin adaptor on one PC (using WIDCOMM) and a Sony VAIO adaptor on my laptop so I can mess around with BT programming using the MS drivers. The problem with the MS compatible adaptors is that they don't support all the BT services like handsfree, headset, etc. If you're just a consumer then get any adaptor (like the belkin).