I started on a 24 but i was getting frustrated & I was terrified of getting hurt in a UPD (UnPlanned Dismount). So I got a 16, it gave a big boost to my confidence. Then I got a free 20 and rode that before going back to the 24.
I have some paralysis in my legs (need AFO braces to walk, see avatar) so everything took longer. If I could learn, ANYBODY can learn.
Timeline, I averaged 1 hr/day, 5x a week.
Could ride the 16" 150 ft @ 2 months
Moved from 16" to 20 @ four months
And back to the 24 @ 6 months
Rode off my first curb @ 8 mo.
Did my first Muni @ one year.
Road SIF, SIB, & backwards @ 2.5 yrs
Each wheel took me about 30 min to adjust to the basics, and after I was comfortable on each it could take me up to 5 min to re-adjust to one of the other sizes.
I still can’t hop well enough to be any use, or free mount.