Depends as much on the mouse as it does on the batteries. I use eneloop rechargeable batteries. The Logitech mouse I had only used 1 AA battery. Even with turning it off between uses I could only go 2 or 3 months of all day use before swapping out the battery. Got a Tecknet mouse that uses 2 AA and haven't had to do a battery swap for the year that I've been using it, and I never turn it off.

I have a Logitech M310 wireless mouse at home and work, they use 1 AA battery. I get at least a year out of each Duracell battery.

Same mouse here, with similar battery life. Using it 40 hours a week, and never turned off. I also have a USB extension cord coming up under the monitor from the computer its self. I'm sure that helps some as well since it's only transmitting about a foot, versus the three or so, and through a 1 3/4" slab of wood to the port on the computer.

Got my results from running two Energizer Ultimate Lithium AA batteries in my Magic Mouse.
Just over a month. Of note, just last week was the first time I saw the batteries showed to drop from 100% to 97%. This morning they were at 10% @ .78 volts and would not keep the mouse turned on all the time. My eneloop AA batteries would keep the mouse going till the monitor showed they had dropped to 4% and the battery voltage was .68 volts.
My thoughts on this is that the rechargeable eneloop has a lower internal resistance so it allows more current and less voltage drop although the mouse does not use much current.

Now I will try the eneloops again to see how long they last exactly. I have do not know exactly but will post again when they get too low to operate.

I won't change from the magic mouse anyway. I like it too much. It has almost no moving parts, all the functions are internal and its small and easy to use. Wipe your finger up and down or side to side to scroll. Right and left click is the only moving part.

Class A review right there!

Science and s*** even.

Ok Apple Magic Mouse after 9 days. It is down to 10 percent this morning and will probably stay up all day but I will change the batteries out this afternoon or evening.
So the Magic Mouse pulls a fairly high current versus most other mice but I don't care. Its more comfortable to me.

The 4 year old eneloops hold up about 1/3 of what a disposable lithium battery does in this application.