I was teaching step in the early 90's and had about 7 years' experience teaching floor aerobics at that point. As I recall, it took me about 6 months to get pretty good at breaking down and cueing my routines when I first started teaching floor aerobics (that was with teaching 10-12 classes per week and doing a lot of practice at home). When I started teaching step I'd say it only took a month or so to get to a good skill level because the basic teaching skills are the same, just the moves and the names of the moves are different, and I had to learn how to give a few beats' more warning for move changes on the step.
Stebby