If you have a load of X amps, and your time to your cutoff voltage is Y, then you just multiply them together to get your amp/hour rating. Multiply that by 1000 to get your milliamp rating.
Example:
You have the battery giving a current of .5 amps.
It takes 3 hours to get to your voltage stop point.
Multiply 3X.5 to give you an amphour rating of 1.5, or 1500 milliamp hour.