I had a Gameboy Advance years ago so don't quote me, but I agree with online sources of about 8hrs or so. I would strongly advise on not getting one though since the screen is really hard to use. The lack of a backlight is infuriating and even in direct sunlight, you barely see much of the colors.
Unless you really want a GBA, either get a Gameboy SP or an original DS. I doubt either would be much more expensive but are heaps better to use with the backlight and re-chargeable batteries (with the bonus of charging from 5V so can use USB charging cables for them). I'm sure the extra cost would be worth it.