I recommend reading the “Using my WaniKani Knowledge” section in https://www.wanikani.com/guide and using it to help create your study-plan, expecting each level after 3 to take a week and a half to two weeks. (And read the rest of the page because it’s filled with answers to what really are frequently asked questions)
I came in with about a thousand vocab words alongside a working-but-limited understanding of basic grammar and was able to start reading Yotsuba without too much difficulty after completing Genki I and hitting level 8 in WK (with the English version, which I had not read before, handy as a reference). After WK level 11 and finishing the second volume, there have been very few words I’ve had to look up (3-4 per chapter) and my reading speed went from like four hours for the first chapter across three nights to about a minute per page (except for the movie-dialogue one). It’s very tough at first, but if you maintain a sustainable studying pace (even if it feels slow at times), within a few months, you’ll start to see real progress for sure.
If you’re coming in with a lot less prior knowledge than I had, don’t worry: I’m on the brink of hitting level 15 (after about four months) and have already encountered, in WK, over 90% of the vocab needed for Yotsuba and other easy-end-of-the-spectrum stuff I’ve tried reading, enough to make educated guesses about unknown words in context. Just don’t slack on grammar after you start studying it and you’ll find you really can understand a little bit more with each passing day. (Note: Yotsuba’s dialogue is very hiragana-heavy, rather than kanji; I find it helps with recall, in the “given this reading, what does it mean?” sense, and with reconstructing conjugated verbs, which has provided a noticeable boost to listening comprehension. The older characters speak with kanji that matches their expected education levels, and it’s now uncommon that I even need to read the furigana next to it, since the WK SRS is doing exactly what it’s supposed to)
As someone else in the full-time-job situation, I know the journey to fluency will take years, but the more I read, the more I want to continue, and I’m sure you’ll feel the same after you read your first page of text.
Note that I don’t specifically advocate using Genki. It’s what I started with and it teaches things really well, and WK goes out of its way to prepare you with kanji and vocab that are specifically covered in the textbooks, but I’ve since switched to Tae Kim because his style is more in-line with how I learn. Any reputable grammar resource will get you to the finish line; finding one that works well for you will just get you there faster.