I can't imagine any sort of procedure on one tooth that would take that long. Please verify with the dentist themselves, not the office staff, the dentist.
My granddaughter fell and broke her front tooth off and had to have a cap put on it, she also had some other cavities. She had work done on 4 or 5 teeth all together and wasn't under for over 45 minutes.
My grandson had gold caps put on a couple of top rear teeth, a couple of fillings on the lower molars, and a couple of other teeth with little spots that were the start of a cavity that they just wanted to smooth off. He was out a little over an hour.
So, I don't know what they'd be doing on one tooth that had to be done all at once and could not be broken up into smaller surgeries.
If this were me considering having this done on one of my grand kids I would try to understand what the procedure is, why it takes so long, what can be done to shorten the length of time such as doing several different days of anesthesia, etc.....
DO NOT DO THIS WITHOUT THE ANESTHESIA. It IS so much easier on the kids and on the parents when it's all done. The kids wake up, ready to go, they don't have any side effects such as disorientation or time for it to get out of their system, they just sleep until it's all gone then they wake up fully rested and ready to go eat and play.
I am really hoping you'll expand what the dentist is planning on doing in that time frame. I do know that many kids have cavities and it has absolutely nothing to do with brushing, flossing, diet, etc....it's genetic or environmentas.