It depends a lot on the quality of the batteries you use, how often you use the toy, and whether or not you remove the batteries while the toy's not in use (doing so will extend the life of the toy somewhat, as well as the life of the batteries).
How long do the batteries last, does anyone know ????
I also store my toys without the batteries. They last longer and don't. Leak. With this toy I tend to use it for 2hrs at a time so they last way longer.