“How long does pinhole gum surgery take?”

Not too much time, it depends on the procedures.

If you are having the surgery, have you not asked the doctor who will be doing the surgery? It could take as long as an hour or two or as quickly as less than an hour, depending on how much is being done. Make sure you talk with the doctor who's doing the procedure before you go in for it, as informed consent means getting all the information about it, including
risk/benefits, expected outcome, etc.