How power efficient are Apple's bluetooth devices?
I have a Magic Mouse and a Wireless keyboard, and I use rechargeable batteries. I charge them once a month, but I'm sure they'll last at least one more. 

All the recent Apple wireless (Bluetooth) products are very smart about power usage and it really doesn't matter wether you turn them off or leave them on. I turn them off only when transporting them in my bag.

Note that the battery percentage reported for bluetooth devices is not accurate. It's based on the voltage of the cell, and since it's different in the Alkaline (1.5 V) and the Ni-MH rechargeable (1.3 V) batteries, it's virtually impossible to distinguish between the two. Not to mention the discharge curve of both.

Changes in 10.6.7 made it more accurate for devices, running on rechargeable batteries - It now treats charge around 1.3 V for 100%.