Embr Wave is designed to give about 15-50 sessions of warming/cooling.
This translates to about 2-3 days of intermittent use according to existing customers.
The actual battery life will depend on the environmental conditions, intensity of warming/cooling sessions, and frequency of usage.
Li-ion batteries can see their capacity go down by 50% in very cold weather (you might have noticed this with your smartphone, too). Embr Wave is designed to operate in moderate environments. If you're using your device in very cold or very hot environments, that's okay, but it does mean it's probably using more power and depleting the battery faster.