mAh = milliAmpere hours.
If you have a device with a single AA that takes 1mA of current then a 2500mAh battery will last 2500 hours, and a 1300mAh battery will last 1300 hours. At 10mA current draw the life will be 250 hours and 130 hours, and so on.
Theoretically, as Knuck says.