Battery life will vary based on whether you prefer the "constant on" or "flashing" mode. Flashing mode uses about half the power, so you get close to double the battery life.
Our current model uses (and includes) 3 AAA batteries, which can last you up to 40 hours in flashing mode and 20 hours in "constant on" mode with high quality batteries.
You can replace the included alkaline batteries with rechargeable batteries if desired.