How long does preparing to ship take for iphone 12 pro?

From my personal experience the Preparing to ship can last a day or two, the when it's officially shipped, you'll receive a new confirmation that is has shipped with the ability to track the order. You certainly can get the phone earlier. It will typically arrive within the date range you were told at order. Example, if you were told your phone will arrive between December 4 and 10, it can come anytime during that time frame.

When you placed your order you were given a delivery date.

You also received an email confirmation with the delivery date.

Your phone will arrive on the delivery date you were given.

I ordered mine last Nov.13 still in processing status. Promised delivery date is in between Dec 3-10,2020.

I have personally received Apple products before the estimated delivery date, but it was just one day ahead of schedule. Sort of hoping that's the case with my most recent order :'-(