Additional screen time allowance request: when does it time out?
The answer is: when your kid requests additional screen time for the app xy, and you grant it, the additional time starts at the moment you allow it. So it's not extending the user's screen time at his/her discretion. 

Example: 

Your kid has used up all his/her screen time for the day, but asks for 15 more minutes of camera and youtube. You grant it (via Screentime) at 15:00. At 15:15 both apps will time out again.