Points (between any two programs) don't take a fixed time to transfer, they only tranfser at certain times. So "how long" is the wrong question; "when" is the right question.

Amtrak transfers to Choice on Thursdays (so show up on Friday), but obviously, only Thursdays that are business days, since last week no transfers happened, and Thursday was a holiday.

FWIW, such transfers are often not simple. It's not a matter of pushing a button or running a script. Many partners transfers between themselves using physical media of some sort (tape, disc, etc) through some sort of physical delivery system. Takes people working that day at both ends to do it, and their schedule may not allow for it to be done on a different day if the day it normally happens is a holiday in a given week.

Furthermore, because it's physical media, sometimes it gets corrupted. In that case, add another week!

(I know nothing specifically about how AGR to Choice transfers happen, other than the timing. I do know that some other trnasfers between other pairs of partners definitely happen by physical media being shipped, though.)