Purely from the numbers standpoint, if you added 2 new grammar points a day, you could finish adding N5 thru N1 in one year (12 months 3 weeks)…
And if you include the time to reach Level 12 on all grammar points, then you’re talking like 2 years.
…BUT, but, but, as others have said, studying grammar is a different beast than studying vocab. Even that leisurely pace has to be paired with something else – reading, tutors, videos, blogs, etc. It takes time and practice to have the concepts sink into your brain, and some concepts are definitely harder than others.
Given the data they have, it would be interesting to see something like 1) the average time people take to finish adding all the points from a given N level, and 2) the average time to reach Levels 9, 10, 11, 12 from that same N level.