I'm going to pick nits just a bit, just to get it out of the way.
In many cases, the "average" length of time is meaningless, if it's even really known. Back in the association I was in, I could tell you how long it took me (about 12 years), how long it took the person who got theirs the fastest (which is also the minimum - about 3.5 years), and that most folks who get there take at least 6 years. But the range is pretty wide. Other than the one guy, I know of nobody who got theirs in less than 5 years, though I expect there were some. I know of nobody who took as long as I did, aside from others who had some sort of interference (my business travel, a pilot with even more travel, etc.).
So I could see an answer being more like, "It takes a minimum of 3.5 years of constant training, but most folks take well over 5 years." Which doesn't really give much information, except that it won't be quick. I only had one person ever ask me about this in my curriculum, and my best answer I could give was that I didn't think it was even possible in less than 7 years, unless someone came in with a lot of NGA experience from the NGAA.