Assuming you could devote, say, 4 hours everyday, or at least 6 days per week, how long would it take you to learn HTML, PHP and CSS?

My recollection is that you don't reside in the U.S.A., which could easily knock even the most sympathetic or favorable advice out of whack (into uselessness).

Should I assume that you're expecting that what you "learn" will lead to more profitable employment?  I concede that it's an imperfect analogy, but are you seeking the level of skill akin to a lumberjack who might be able to build a rough-log cabin, or to a common carpenter who might be able to reproduce Early American furniture, or to an artisanal carpenter who could design & build elegant structures like a traditional Catholic altar?

What technical background do you already have that can provide a basis for your new effort?

You do know, don't you, that idiosyncracies of the software industry have already led to plenty of bad software "out there",  and yes, I absolutely positively do include Microsoft.

There are several developers among the CathInfo membership, including myself.

All I can say is: the industry is ruthlessly competitive. Unless you have a strong interest AND a natural talent for some kind of engineering (building stuff, particularly in the abstract) you won't succeed.

There are so many in the industry already, that employers assume you know nothing and spend a LOT of time testing and grilling each applicant, even if you have a long resume!

I can't think of another industry that is SO BAD about not only your KNOWLEDGE becoming obsolete every 5 years, but even your experience! It's almost ridiculous. I have literally done nothing else career-wise since I was about 21 (except 3.5 years at a Trad seminary) and I couldn't find a job for about 6 months last year. I had plenty of applications and interviews. I felt like a useless failure. And I DO have natural talent along with an enjoyment of programming, plus I've developed that talent. But anything I did more than 4 years ago was completely worthless.

There are several developers among the CathInfo membership, including myself.

All I can say is: the industry is ruthlessly competitive. Unless you have a strong interest AND a natural talent for some kind of engineering (building stuff, particularly in the abstract) you won't succeed.

There are so many in the industry already, that employers assume you know nothing and spend a LOT of time testing and grilling each applicant, even if you have a long resume!

I can't think of another industry that is SO BAD about not only your KNOWLEDGE becoming obsolete every 5 years, but even your experience! It's almost ridiculous. I have literally done nothing else career-wise since I was about 21 (except 3.5 years at a Trad seminary) and I couldn't find a job for about 6 months last year. I had plenty of applications and interviews. I felt like a useless failure. And I DO have natural talent along with an enjoyment of programming, plus I've developed that talent. But anything I did more than 4 years ago was completely worthless.

When you have 2 guys with "3 years relevant experience", one guy is 20 and wants 50K a year and the other is 42 and has a family, and wants 80K, guess which one most people hire? The first. That's because they're cheaper, younger, more willing and able to put in long hours, more likely to be liberal -- plus the younger ones have less attachments, have no family except their co-workers (which tries to be a pseudo-family).

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FYI I'm not a coder, so my insights are more general:
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Keep in mind that corporate big wigs have a vested interest in hitting the "we don't have enough programmers" button.  Why?  Because people like you believe it, go out and learn how to code, and so do tons of other people, and then you all hit the job market and they can pay you pittance because now (after they've used their influence) there's tons of programmers.

Programming is also a little tricky, I think, because only other programmers can tell if a programmer is any good.  It isn't like (say) interviewing to work in sales, where even the HR manager who might not have much exposure to what the sales staff actually does, can make perfect sense of an internal job description.  All an HR manager can do in the case of a programmer is take your word for it that you know Java, SQL, Python, or whatever.  He can't actually tell.  He doesn't even know what these things are except that they have something to do with software.  Point being, the selection process from one company to the next is probably going to be a bit wonky because it's more difficult for the decision makers to all get on the same page.

The "gig economy" is growing for all industries, and definitely for programmers.  But keep in mind that "knowing how to code" is only one prerequisite for the gig economy for coders (or anyone else for that matter).  You are essentially a sole-proprietor, and as someone who has been part of the gig economy (or someone like Matthew, who has coded in it) can tell you, that means you have to market yourself.  You have to get along with other people.  You have to have good business sense-- you have to know how to communicate with others, how to sell your skills, how to anticipate needs, how to resolve conflict, how to invoice people and make sure you get paid, and so on.  You'll be doing way more than "just coding" in the gig economy, you'll be running a business.
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I say all of this not to discourage you or invite cynicism, but to hopefully paint something of a clearer picture of what to expect.  Obviously people are successful as programmers, and people do make a living in the gig economy.  But it isn't going to necessarily just be a matter of going to code boot camp and then getting hired for the first six figure salaried job you apply for.  There's gonna be risk and you're going to likely have to climb proverbial ladders.