When I tried to get used to a fiver it took a month or two until I was comfortable. But when I would play my four stringers again, my spatial memory had been rewired and I was stumbling across the fret board on the four. I had to play the four for a few days to get used to the original low string anchor point again.
After about 6 months of going back and forth, I realized that my brain works best when I stick with one or the other.
Different musicians think differently when playing, and my thought processes make it so that it would take a very long time to be fully comfortable switching back and forth between two similar, yet somewhat different, feeling instruments.
It's the anchor point that trips me up. I have 20,000 hours over nearly 40 years of playing with the anchor being the E note 4th string. I can't just flip a switch in my brain and automatically become totally comfortable with a B note 5th string.
Since I don't envision having a real need for more than 4 strings with the type of music I play, I just decided to give up on the fiver.