The amount of energy a radio is using is the current. A battery is typically rated by how much current can be drawn over a specific amount of time. The more current a radio is pulling, the more energy is drained from the battery. A radio uses very little power while turned on doing nothing, but when receiving, more power is used, and a substantial amount of power is used to transmit. For example, if a radio that pulls 50 mah was tuned to a local weather information loop, and was always receiving, and the radio had a 1000 mah battery you can figure out approximately how long the battery would last.