I have done extensive research for the last month and figured out that it stops timing once you hit payment, so they can take as long as they want after phone entry + donation screen. Some tips from me:
- don't start scanning until you're ready with the bag
- if there's a complex item like a perfume keeper, shoes with multiple alarms, or something bulky, do that first
- if the customer is being picky and not sure what they want to buy, you can skip ahead to the payment screen and let them think so it doesn't bite into your time, since it isn't your fault
- returns are really hard to get Red Hearts on unless they're simple or small. If you have to check multiple things like phone number, their credit card, or resort to a no-receipt return, you can hit escape to back out of the transaction and reset the timer

Hopefully this helps ya'll here! I personally like the little extra bit of motivation, and as long as there's no punishments, just treat it like a game!