The battery we chose for this project is a large 6600mAh battery sold by Adafruit. We measured the power consumption of the entire device to be ~15mA, so it should last 440 hours, or about 18 days on a full charge. Below is a discharging curve from a device in a cage with a mouse, showing that it lasted 14 days from fully charged (this is close enough to 18 days to satisfy me that our calculations were correct).
A 2 week battery life is acceptable for many applications, but is not ideal. On the bright side, we achieved this battery life with no sleep modes activated on the processor. We have been experimenting with sleep modes and believe we can extend the battery life by ~5x with updates to the code, but we don't have that ready to release just yet.
However, searching online, a PIR based motion detector should be able to last MUCH longer - Texas Instruments has a reference design that they claim can last for 10 years on a coin cell battery. In future iterations of this device we may move to a design like that.