A 'teacher' who only knows one social style is not much different than one who knows two. Length of time is never a criteria - it's how much time was spent on technique and how much time was in reality 'dancing around'.

Many folks will tell you that they've been dancing for (say) ten years - but that might include 20 group lessons and years of social dancing with the wrong technique. Most folks don't want to hear this (they think that the learning process is very fast in dance and that they also think they can see who is 'good' or 'bad' simply by observation in a social situation) and therefore see a 'teacher' who's been doing social teaching (for instance) for ten years as having more dance training than, say, a teacher that has extensive dance training but only a year or two of ballroom.

Than, there's 'ballroom' and there's BALLROOM, but that's a topic for another thread....

I can only speak from my own experience over a number of years. After dancing for many years with a teacher who taught
three different styles of dancing, I went to a Latin specialist who was a former top Professional in Europe, seventh in the world actually. What she was able to teach was unbelievable. For instance , did you know that very few people are capable of dancing the Samba to the correct timing because of its changing rhythms. Only a highly trained specialist can do this. Believe me.