The companion progression would feel more natural, as time flows very fast in Morrowind.
Yes, it's a straight 5 days no matter what the circumstances. They just have to be your companion. Once the companion is one level below the PC, they won't level up until the PC levels up. So the time it takes a companion to level will be the same as the PC but with a minimum of 5 days.
In this case, the PC is 16 levels above the companion. It will take the companion at least 75 days of game time to reach level 19.
I picked 5 days after reviewing a number of my characters. I found the range to be 3 to 10 days average per level. The lower level characters leveled up more quickly; the higher ones, more slowly.
I could change the level up to occur more or less frequently depending on the difference in level between the PC and the companion. With a greater difference the companion would level up more quickly. As the difference in levels decreases, it would take longer. This may be a little more realistic with regard to training, but it won't make much of a difference in the time it takes the companion to reach the PC's level.