Brightness level 1 (lowest): 15-19 hours.
Brightness level 2: 12-15 hours.
Brightness level 3: 8-10 hours.
Brightness level 4 (highest) 5-8 hours.
Flashcarts tend to shorten those amounts by as much as 15%. Volume also affects those numbers.
In each category, the lower number is DS games and the higher number is GBA games.
Those numbers also assume that the battery is working at full potential.
Hope that helps ya!