good luck with your appointment with the surgeon.
How long till you can be booked in depends on the
surgeon and his process. There are usually dietician
appointments and possibly counselling sessions as well.
Take the time to prepare yourself mentally for the journey
you are taking. It is difficult physically but also mentally.
I would suggest waiting to see the surgeon and get a date
before starting the preop diet, you don't want to be stuck on
the diet too early or for too long.
All the best.