One thing that sticks in my mind from that long thread: Michael’s advice is not to bother with marketing until you have at least three books published, all of traditional publication quality. Then the ratio of marketing to writing should increase. Eventually a split of somewhere between 30-70 and 50-50 might make sense, but I don’t remember if that came from Michael.
The amount of time you spend on the business and marketing aspects of writing will depend on whether you choose traditional or self-publishing or a mix, as Michael did. Either way, the author has to engage in marketing, but more for self-publishing once those three or more books are out. Trad helps with marketing pre-release, and manages the product in such a way that the only book worth marketing is the one just about to be released. With self-publishing, it’s an all-the-time pursuit, because it helps all the books all the time.
The other business aspects of publishing - market research, cover art, blurb, interior design, distribution, print, advert prep if you do that, finances - require little author time for the trad route. After all, the trad-published author has sacrificed most potential future revenue so that the publisher will take care of the effort and risk associated with those things. For the self-published author, they require a lot of work and some outlay of cash. Michael took an innovative approach for print: he raised money for an offset print run through crowdfunding.
For self-pub, the amount of time you spend on business vs. writing will also depend on your product model. If your product is ‘high volume, low quality’, then you probably will benefit from spending the greater part of your time marketing.
If you write high quality at lower volume, but have devoted fans, you’d want to spend more time satisfying them (that is, writing), and let them do more of your marketing for you, by telling their friends, writing glowing reviews, etc. For the aspiring self-publisher who dislikes the idea of marketing, as many do, a sensible path might be to gradually find that style that captures the hearts of a dedicated core audience, build it up, and keep feeding them, while they get the word out. Takes time, talent, engagement, and market sense, of course.
The answers I’ve see across the spectrum of ‘professional’ models ranges from about 90% writing (trad, successful, takes some time to manage brand and socialize at launches, or just starting out), to 95% marketing and production (use fiverr ghost-writers to bang out ripoff self-help books by the dozen). So there’s no one answer.
The above is a a personal distillation of the views and info from others, probably colored by my own opinions. Each must find her or his own path.