Well, we now have an answer to how long it will take to move:
One of them comes from GamesBeat's Jeff Grubb who tested how long it takes to transfer files between various drives. In the video embedded above, Grubb moves Assassin's Creed Origins (which is a 49GB file) around his storage locations in different configurations. Here's what the tests found.
Moving Origins from an external SSD to an internal SSD took only 2 minutes and 18 seconds. The inverse, going from internal SSD to external SSD, took 4 minutes and 33 seconds. Probably most important, transferring from an external HDD to an internal SSD took 7 minutes and 46 seconds. Finally, moving from internal SSD to external HDD took 10 minutes and 36 seconds.
That third result is the one most people will want to keep in mind. It's the one that dictates whether they can live with swapping installs back and forth or whether they'd rather pony up for the official external SSD. Approximately 8 minutes to get the game where it needs to go in order to be played. Assuming next-gen installs are much bigger -- let's just call a file 100GB for convenience's sake -- it'll take about 16 minutes to transfer a game.