I'm on ze list but not a beta tester so haven't used it, but as I remember the claim was to expect up to a couple of years life with moderate use; works out about $40 a year so seems ok to me if I mentally model it as a subscription.
Update: ah, here (obv the founders claim):
How long does the battery last?
Roughly 12 to 15 hours of recording. On average, I use it 10-20 times per day to record 3-6 second thoughts. That's up to 2 years of usage.