Too long
As icefang97 already mentioned, WK only teaches kanji and some vocab to reinforce the kanji readings, it is by no means a comprehensive Japanese resource. If all you do is WK, at level 60 you’ll be able to recognize a lot of the most common kanji (but not all) and know some of the vocab that uses it (but not all, as that is not the purpose of this site. Words commonly written without kanji would also be quite difficult if you only ever learned the kanji variant).
As far as kanji go, as this is a novel intended for a mature audience, they’ll probably expect the reader to know the 2000+ kanji students learn through junior high school. Only uncommon/exceptional readings will likely be written with furigana the first time they appear.
Depending on how much having to look up unknown words/kanji bothers you, and how big your tolerance for understanding less than 100 percent is, the threshold varies of course.
You can start reading at any time. Depending on your knowledge of vocab, grammar and kanji, as well as your expectations regarding comprehension, the time it would take to read it will differ greatly, of course.
That said, unless you’re particularly stubborn in the face of difficulty, this should probably not be your first native reading material. Imo, of course.
Raionus helpfully put the amazon sample into this site that acts as a browser reader, letting you look up unknown words and kanji as you go. (As I understand it.) Might be useful in gauging the level of kanji and grammar needed to comfortably read this novel.