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Could be the type of batteries, or if you're leaving your mouse on 24/7,
try turning the mouse off when you're not using it.
 
that sounds a bit extreme, look into rechargeables. My Mouse and trackpad are both recently new but the batteries aren't going nearly that quickly.
 
Using Apple rechargeable batteries, my Magic Mouse runs for 4-6 weeks before the batteries need replacing.

Both the mouse and the iMac are left on 24/7. They both sleep when not in use.

There is no need/advantage in physically turning the mouse off when not in use - it does this automatically.
 
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