I've heard that you can do it in a day. You just have to basically not sleep a full night, which will tire you out enough to sleep at a desirable time. Then obvs make sure you wake up, or are woken up, at a desirable time asw.
If you have some more time, then definitely try to do this more slowly. E.g. waking up 2/3 pm first, then working your way to 12/1 pm etc.