The process was swift and the time governed only by the number in the group, rarely more than 4 or 5 in my case. If facing the Headmaster alone, I suspect it lasted no more than 10 minutes (except on one memorable occasion). If it was one of the senior masters it could take longer as one of them used to make one of the pupils walk from his office on one side of the school, to the secretaries office, adjacent to the Headmaster’s office, to fetch the punishment book and a cane. Now that was a long, slow walk.
If guilt was not admitted or there was an element of doubt, there could be a delay while further information was sought by the Headmaster, but it was unlikely to go into the next day.