My Logitech itouch wireless keyboard has been on the same set of batteries for 1 year, and it still works fine.
This computer is on 24/7 and I use it 14 hours a day typing up code for work.
My microsoft intellimouse wireless on the other hand eats through a set of batteries in about 2 to 2.5 months.
Also the keyboard is using cheap generic batteries, the mouse uses duracell ultras. The generics lasts about 2 to 3 weeks in the mouse.