The length of time that a battery operated lawn mower lasts will depend on several factors, such as the size of the battery, the capacity of the mower, and the type of grass being cut. Generally, a fully charged battery can last anywhere from 30 minutes to an hour, depending on these factors. However, it's important to note that the battery life can be affected by other factors such as the age of the battery, the temperature and humidity, and the terrain being mowed.