My beginners (even though we don't exactly classify them like this)
have a range of about 4-5 years commonly. It's the same for the other levels as well. Much of this comes from how much learning they've experienced outside of their regular schools and (I hate to put it this way) but how smart and determined they are. I have had 10 year olds taking English class with 13-17 year olds and not struggling at all.