To calculate high demand slots, the system adds up all the orders you have accepted during high demand slots over the past 28 days. Keep in mind that it is when you accept the order that matters, not when you finish it. This means that, if you accept an order during a high demand slot, but finish it after the high demand slot ends, it will still count. If you accept an order before a high demand slot, but finish it once a high demand slot has started, this won't count.
If you are inactive for 7 or more days, the app will understand that you are having a break and will keep your score due to high demand slots until you are back, so your Excellence Score won't drop.