I guess the question is how long it should take to become an instructor. Currently an ambitious beginner with little relevant experience can become a competent dancer in 1-2 years if they are willing to put the time in and dance a lot, and have access to competent instructors. I don't think it is unrealistic for them to become an assistant instructor in another 1-2 years if given training on how to teach, and gradually progress to leading a class themselves.
I agree! The salient parts are "put the time in, "have access to competent instructors," and "given training on how to teach."
I wasn't super-ambitious, but I was in classes between 4 and 6 days a week from early 2013 to mid 2017, at first as a student and then repeatedly in the same classes as an accompanier. During that period, I also took private lessons with one, sometimes two, and occasionally three teachers a week, with two different working partners. I went to at least two group prácticas a week, and rented studio time for private practices. Near the end of those 4 years, I was trusted to be a teacher's stand-in demo partner, to give the free intro-to-tango classes, to be a substitute teacher for the occasional all-levels open class, and to organize/lead a práctica.
Based on my experience, I'd say that 2-4 years to assistant instructor is quite reasonable. In my case it took 4 years, but as I said, I wasn't super-ambitious . . . everything happened by chance, in that others picked me, and I was in the right places at the right times without making the least effort to be there. Also, I was on the old side for a new dancer, starting in my late 40s. An ambitious person in their 20s or 30s could easily do it in half the time.
However, a few people who started when I did and now pass themselves off as teachers didn't spend all week in classes, take back-to-back private lessons, rent practice space and practice, or phase themselves into teaching as substitutes and stand-ins first. They took a few years of group classes and then declared themselves teachers. I don't think anybody is "talented" enough for that to make sense.