You will hear the available talk time when you turn on the headset, as well as at the end of each call.
Check the Battery Level in Between Calls:
Check the Headset Battery Level on Your iPhone:
iPhone users are able to check the battery level right on their phones. The headset battery meter appears in the upper right corner of the screen, to the left of the iPhone battery meter (see below).
The headset provides up to 4 hours of talk time and 7 days of standby time. It takes 2 hours to fully charge the headset.