Once the case has been fully charged, it can provide four full charges (0-100%) to the earbuds, giving 36 hours of total play time, in addition to the charge inside the earbuds.
If you are unsure of how much charge is remaining in your case, there are five LEDs on the front which each represent 20% battery charge when the case is open.
For example, two solid LEDs means the battery case currently holds a 40% charge whilst all five solid LEDs indicates the case is fully charged.