The Magic Mouse comes with AA alkaline batteries pre-installed and ready for use. Some early Magic Mouse users report that battery life is abysmal, though: a mere 30 days or so.
Courtesy of Apple
The batteries, and not the mouse, may be the culprit. In most cases, the Magic Mouse comes with Energizer batteries, which are a well-respected brand, but it's hard to know how long they've been on the shelf before being installed in the Magic Mouse. It's likely that new, fresh batteries will last longer than the 30 days some users are getting out of the initial batch.
Of course, battery life also depends on usage. The Magic Mouse is supposed to go into hibernation when it detects a lack of use, which should help extend battery life. Turning the Magic Mouse off manually when you're done using it, with the switch on the mouse's belly, should help push battery life a little further.
Another option to get the most life out of the Magic Mouse's batteries is to replace them with either Lithium-Ion AA or rechargeable NiMH (Nickel Metal Hydride) batteries. Both should provide longer life; the NiMH batteries have the added benefit of being rechargeable.
If you decide to go the rechargeable route, look for NiMH AAs with a 2900 Mah (Milla amp hour) rating or better. Many of the bubble-packed, brand name rechargeables you find in the checkout aisle of your local hardware store have a 2300 to 2500 Mah rating. While they will work, they won't have as much staying power, and you'll find yourself recharging them fairly often. The 2900 Mah batteries are sometimes referred to as 'High Capacity' or other marketing babble.
Lithium AAs are also available in various Mah ratings, and once again the 2900 Mah rating is a good value to look for. The advantage to the Lithium batteries is a much longer battery life than standard alkaline AAs. They also last longer than the NiMH batteries do on a single charge, but they're not rechargeable.
Let us know the battery life you're seeing with your Magic Mouse.