Like most things if you have the basic information the calculation is easy. First you have to know the average current you are using from the battery (in Amps), then the capacity of the battery (in milliAmp hours).

Divide capacity by current, multiply by 60 and that's how long the battery will last in minutes.

As you see we can't be much more precise without some idea of what battery you're talking about and what you are using it for. I have batteries with capacities from 50mAh to 3000mAh and systems with average currents between about 100mA and 30A.

Steve