This depends on the activity in your home. With an average use you can expect the batteries to last for approximately 2 years.
As soon as a sensor is disconnected from base it tries to reconnect itself. If this happens often or takes a while, it can reduce the battery life. Regarding this the first delivered sensors had a bug in the firmware. This bug has been fixed with the update from November 19th 2013 with the result that the sensors should remain firm with base.