I would like to come up with a rule of thumb which will even work for a language you can’t read. Rasana, as a Russian tutor you can grade lessons, but as a Russian learner it’s not as obvious to me. As a Japanese beginner, nearly all texts are too hard for me, what I’d like is an idea of how many months/years it will take me to be able to understand any given novel or web site.
It would have to be based on the number of words and characters per sentence, because you can count them without even knowing what a vowel looks like. It can’t involve counting prepositions, because that assumes you can recognise a preposition when you see it. It would need a different fudge factor per language, which would need determining empirically.
I think some graphs may be involved.