Battery Life can refer to either:
The length of time a battery lasts between charges.
This is difficult to predict because a radio uses a tiny amount of power in Standby, a moderate amount when Receiving a signal and a lot of power when Transmitting.
Because there is no way of predicting how much a radio will be used each day manufacturers make an estimate of battery life (in hours) based on 5% Transmit, 5% Receive, 90% Standby. All radios will last at least 8 hours based on this figure. In real terms this will be achieved with a 1500mAh battery for most radios.
2000mAh batteries are also available to extend battery life but most chargers will let you charge a spare battery while the radio is in use. Gradually the capacity of the battery decreases so eventually it will not hold enough charge for a full days use.
The number of times are battery can be re-charged before it will not last a full day.
This is normally quoted as between 300 and 500 charge cycles by manufacturers - enough for a year of daily charges (holidays and weekends excepted). In real terms we find batteries last around at least 18 months with heavy radio users and as long as five or more years with less demanding users.