You’ll find the battery’s mAH listed on its side. Use this equation to figure out how long the battery will last:
Amp Hours / Amps = Hours or aH/A=H
So a 2,500 mAH battery will last 10 hours in a device drawing 250 mA of current. But the same battery will last 100 hours if the device only draws 25 mA current.
Your battery life ultimately depends on how much current your device draws from it. For example, lower loads make the battery last longer. So a penlight’s battery will last longer than one in a large flashlight.