The length of life that you can get from the batteries will depend on how you ride. If you ride in fair weather using the eco assist and put in more work yourself, then you might find yourself getting hundreds of miles out of a single charge. However, using turbo-mode, using the motor more frequently and riding in headwinds will result in a reduced range. The average range is around 45-60 miles although some more basic batteries may only get 20-30 miles average.
Battery life is measured in mile-hours[mh] the greater the MH, the greater the range.