It's important to recognize that battery life is going to be heavily dependent on how you use your Spectacles, and Snapchat is purposefully unclear about how long you can expect Spectacles to last for this very reason. So, keep in mind your experience with battery life can and will vary.
Like many devices today which work wirelessly in conjunction with other devices, like a Bluetooth speaker and a phone, Snapchat Spectacles are always on so you can connect to and use them whenever you want without delay.
For example, after you take a picture or record a video on your Spectacles on iOS, you'll get a notification on your phone telling you there's a Snap ready to be imported. From taking a Snap on your Spectacles to sending it off to someone, there are only a few taps involved.