The more extreme stories claim to pass JLPT1 in 6-8 months investing 6 hours or so average a day. That's about what, 1-2 new kanji an hour at about 1k-1.3k hour investment? That's 1 hour a day consistently over 4 years.

Obviously that includes grammar and other things too, but grammar isn't that big of a deal when the sheer volume of characters required to know and contextualize takes care of most.