A common question among language learners is how long will it take me to learn
<insert language>?, and the usual answer, for a native English speaker, is to refer to the FSI list, which divides languages into different categories of difficulty and gives good time estimates for serious students to go from 0 to B2/C1 (high proficiency). That is all great and very useful in terms of helping a learner choose a language based on how much time they are willing to put in.
But another question that could be useful for learners of Japanese is to know roughly how much time it takes to achieve each of the different JLPT levels. Specifically, what are the approximate times to go from:
0 -> N5 N5 -> N4 N4 -> N3 N3 -> N2 N2 -> N1
We can probably expect the time to increase between each level. I assume any answer will be based on criteria similar to what FSI used, i.e., experience of training many students. For the sake of answering this question, English (or similar language such as Dutch, Spanish, French, German, etc) is the L1 and Japanese is the L2.
This question is based on a previous question that was directed specifically to Spanish. Since Japanese is quite different from Spanish, it is worth having a separate question.