Recently a MacFixIt reader wrote in mentioning a problem people have been having after updating to OS X 10.6.7, where Bluetooth devices (namely Apple's wireless keyboards and mice) are being reported as having 100 percent battery level, even though the battery levels were at lower percentages immediately before applying the OS update.
Some initially suspected that the problem was a bug that always reported battery levels at 100 percent, regardless of the actual charge level. However, over time some people have found that the levels have dropped. In some instances the charge drops fast, but others seem to be reporting a more reasonable charge range.
These reports indicate a potential problem with how the battery levels are being calculated. The latest OS X 10.6.7 update did not include any firmware updates for wireless keyboards or mice, so these devices should be using power at the same rate as they previously were, especially under constant use. The only difference could be in the way the OS X Bluetooth drivers are managing the devices and handling the battery level calculation.
If you are having problems with your battery levels not being reported properly, try using your mice until the batteries run out and the system issues a warning (keep some spare batteries handy). This might take a number of weeks, but should help you gauge whether the battery is truly being reported properly. Another thing you can try is to unpair your system from the Bluetooth devices, and then resetting the devices (even changing out the batteries with freshly and fully charged ones) before paring them back up. Lastly, if you have any third-party input management software such as mouse drivers or keyboard utilities, try uninstalling or disabling them, as they could be a possible source for interference for the Bluetooth drivers.
Ultimately this issue is likely a bug with Apple's Bluetooth drivers, but it should not affect the overall battery life of your Bluetooth devices. If you expect your batteries to last between four to six weeks, then that time frame should still be the same, regardless of what the OS says.