I would be interested to know how much% of the battery at the mate 20 per the Always on Display pulls, if I let it run about 15 hours a day?
Samsung itself says under 1% per hour.
I do not think that you consume too much, but I would estimate it at 5-10%.
Here, however, the question arises, if you should use the AlwaysOnDisplay for this case. In my opinion, this is an unnecessary gimmick. Besides, I think it's not good for the screen when it's burning all the time.
Personally, I made that directly after the purchase. It's just unnecessary.