Yet another couple data points for y'all to ponder:
I requested 10K of CO miles to be transferred to Amtrak Wed. morning by phone.
That transfer occurred Sun night/Monday morning in accordance with others' experiences. I immediately requested Amtrak to Choice, but that has not occurred yet as of Tuesday morning.
Monday I also requested the Choice to Southwest transfer for 25K Choice points (I already had those points), and it posted Tuesday morning.
In summary, my experiences confirm those outlined by pshuang.