Watched a couple of BBC documentaries on specialty surgeries. Depending on the kind of surgery and if you are lucky enough to have 2 specialists in the same hospital, you miiiight be able to do a handover partway through. In cases where its not possible, you would try and limit the amount of time the specialist is actually working (eg where they let someone else do the "easy" bits and only have the specialist work where needed). But otherwise it seems 12-18 hour operations arent uncommon. Not sure if this is varies by country, but at least the episodes I saw, if it was going to be crazy long they might try and see if its possible to do the surgery in 2 phases to break it up so the surgeon can have a break in between. But thats not always possible obviously.