Software has been a lifelong passion of mine, going back to around fourth grade. I can't remember what I did first, it was either my mom teaching my Python, or myself trying to learn Pokémon ROMhacking. It's hard to estimate "how long" because I've been learning on and off for my whole life about a whole bunch of different areas.

Just focusing on the web stuff - I was hyperfixated on that for at least a year, probably two years, in high school. It started when I was developing a personal website for a class, I had much more ambitious ideas than everyone else and whenever I didn't know how to do something, I looked it up. Usually the result would come from either w3schools.com, developer.mozilla.org, a random StackOverflow answer, or a random web developer's blog. I'd also be a bit of a perfectionist and test my site on every browser I knew of. I kept developing new websites with even more ambitious ideas and the same process, and eventually got really confident with it.

As for where you could start, that depends on where you are now, but a few tips for general computer stuff:

• Find a way that works for you to learn programming. Al Sweigart has a bunch of free online (and inexpensive paperback) books which introduce the Python programming language in different ways, and one book on the educational game programming system Scratch. There are tons of other books out there to help you learn to program.
• Set up a virtual machine (if your computer is powerful enough to support it) or an old computer or spare hard drive with a Linux distribution, and play around with it. Mess with themes, browse the software repository, set up servers, or anything else you're interested in.
• If you are curious about something, look it up or ask someone who would know! Don't let your curiosity go unresolved. Learning the right search engine keywords is a key skill.

And for web stuff specifically:

• Find a simple-ish website that you like, then right click -> Inspect. See what you can learn on your own just by poking around.
• Install lots of browsers with different engines, open various pages in those browsers, and use their Inspect tools.
• Get a Neocities account and experiment with HTML, CSS, and JavaScript there.