How much power does a wireless mouse use?
Microsoft did a detailed study of this about ten years ago, so the figures given here will be a little out of date.  However modern mice are probably not that much different.

Microsoft was comparing two of their wireless mice against two similar products from Logitech.  Unlike a lot of tests, Microsoft built a robotic device that moved the mouse in such a way as to simulate the way a human would use the mouse.

Studies indicate people use mice on average between 4 and 27 hours per week; but in this study the mouse was stimulated by the motion profile for 44 hours.  The rest of the time the mice were in a deep sleep.

Actually both brands of mice had three power modes: active, drawing the most current, with motion, then no motion for a brief period, then motion again etc.  As already mentioned there is a deep sleep mode, which occurs after about 10 minutes of no activity.  There also is a intermediate sleep mode (idle) which is in between the active and deep sleep modes. 

The measurements were as follows.  Around 30 mice of each type were tested.

             Microsoft Wireless   Microsoft Wire-  Logitech Cord-    Logitech Cord-
             Intellimouse         less Optical     less Click Plus   less Click

Active power
State            11.7 mA             13.0 mA           23.0 mA         22.4 mA

Intermediate
Sleep            942 µA              913 µA            1.32 mA         1.31 mA

Deep
Sleep            157 µA              146 µA            296 µA          293 µA


The tests were run until the batteries (either Energizer or Duracell AA alkalines) fell to 0.8v. 

It is interesting to note that for each of the two mice tested per brand, the current consumption was about the same for each of the power modes, indicating the technology used was similar.

Microsoft didn't say how long the batteries lasted, so I made a calculation using 
the Microsoft mice. Microsoft also didn't say what the split was between the active power state and the idle power state, so I am making the assumption it is about 10 / 90 (10% in active mode, and 90% in idle mode). Given that they were assuming the mice were being used 44 hours per week, and a person is not using a mouse continuously all day long, I think that's reasonable.  I'm also using an average between the two Microsoft mice.

Active mode:        12 mAh * 4 hours * 52 weeks = 2496 mAh

Intermediate mode:  0.9 mAh * 40 hours * 52 weeks = 1872 mAh

Deep sleep mode:    0.150 mAh * 124 hours * 52 weeks = 967 mAh


For a total of 5335 mAh.  Given that two AA batteries in parallel provides 5700 mAh, this indicates the batteries will last about a year in normal use.  Which matches what I get for my wireless mouse.