Generally speaking, the bigger the battery your ebike has, the further you'll be able to travel.
Ebike Battery capacity is measured in two ways. This is in amp hours or watt hours.
Watt hours is a little more precise as it takes into consideration the voltage that the battery is running at whereas amp hours does not.
500 watt hours is a standard battery size. This equates to 14 amp hours at 36 volts or 10.5 amp hours at 48 volt. A 500 watt hour battery will last anywhere from 40km + depending on the motor type, terrain it is being ridden on, suspension system, tyres and how much assistance the rider is giving the bike.
The following calculator allows you to convert milliamp hours (1 Amp Hour equals 1000 milliamp-hours) to watt hours.