How long will doing sealants take at the dentist's office?

Not long. It depends on the number of teeth to be sealed and how easily the teeth can be accessed, isolated and dried. If all goes well it should only take a few minutes per tooth.
Not Long. It usually will only take a few minutes for a dental sealant to be placed on a tooth.
Anxious child. While most of the time it should go very quick, there are times that the child is very anxious and it may take a while longer to just get the child settled and co-operative. I agree, however, the procedure itself should usually only take around 2 minutes per tooth.