Several factors impact this - including how many events your doorbell is capturing, but your batteries may need replacing in as little as a month. They may also last longer. This is why we will soon be pairing all doorbells with Kangaroo Chime.
Why would this happen?
Because subtle inconsistencies and intermittent outages cause the device to “wake up” more frequently and drain the battery faster. While you may not notice this intermittent outages - the Doorbell Camera does.