It is my opinion that there is absolutely no possible way to answer this. And I base this opinion on being around here for a few years and seeing the dramatic differences in time it has taken people to complete the course. That is to say, the few people that I have seen actually finish. I have seen a couple finish it in what appears record, shocking time, maybe a year or less. But for most that have finished the time can be measured in years rather than months.
I suspect that the reasons that there is so much variation in the time it takes can be attributed to a few reasons. There are probably others.
The amount of time available to practice. Or perhaps a more accurate way of saying it is the amount of time people are willing to sacrifice to study. We have lots of people on here that have careers and families and it is difficult to carve out study times. We have others, yours truly included, that are retired and have much time available.
There just is a variable in amount of natural ability students have. We have seen students go on to play in bands. We have others, again using myself as the example, that hope to be able to play along in the music room to a backing track someday.
There is no way to document, but I think most would agree, there is a variable in the amount of dedication the student possesses. This undoubtedly, to a large measure, effects #1. But the amount of fire is going to change outcomes.
Age effects progress. If you notice, I edited this reply. After thinking about it I had to come back and add this one. (Subconsciously, I didn't want to admit this?!?!? 😆) The good news is I and my fellow retired geezers have more time to study. The bad news is we retain things more slowly.
I'm sure there are other factors. I wish that I could give you a quantitative answer but I can't. Sorry.