It also depends on the mouse as well. I use two wireless mice at home. One is a basic HP bluetooth mouse that I cannot remember what the model is, but it was rather inexpensive at about $30. The other is a Logitech Performance MX mouse I bought back in 2009 for around $75 on sale..
The HP mouse uses two AA batteries and it lasts a pretty long time. Assuming I play games 4 hours a day everyday, it can last roughly 3 weeks or 84 hours I think. Probably more than that at the moment I can't remember when was the last time I charged the batteries. The Logitech Performance MX mouse uses a single AA battery and under the same circumstances it probably lasts 5 or 6 days or 20 - 24 hours. I'll need to break out my other Performance MX mouse to...