Carefully and very responsibly follow the recommendations of the surgeon and anesthesiologist. The doctors' recommendations are not their whim or their inventions, and all their work is done for the benefit of the patient. The operating team is a team that works for the patient and if the patient cooperates with the doctors - everything goes as it should! It can be really upsetting when, the day before the operation, you explain in great detail what you need to do this and that, but this and this in no case should be done, and in the morning before the operation itself it suddenly turns out that the patient has drunk a glass of tea. The anesthesiologist understands that the operation must be postponed to a later time, but then the entire operating day and other patients suffer accordingly. And the reason is a drunk glass of tea. You need to be careful and trust the operating team. And besides, there are questions that should not be asked. Most doctors consider this a bad omen (more often at the level of a joke, of course :)) From this series, the question about the duration of the operation. It is customary not to guess :)
It depends on what kind of operation, again. If you have contacts of a doctor / surgeon, etc., you can take a short consultation on this matter from him. I think you won't be denied.