Short answer:
A bit over 6 years.
Long answer:
I started with the general premise and the world building back in early 2013.
It was supposed to deal with the question: "What if comic book characters were just like real people?"
I worked from there and every piece of meta fiction I have seen rise in popularity since then has influenced my world.
Rick and Morty, Community, The Unbelievable Gwenpool, various types of anime from isekai to power fantasies to harem comedies were all taken not only as inspiration but actively contributed to the world I was trying to create.
Around 2017 I began working on some of the characters as well, figured out their motivations and how they would deal with certain situations. I researched various types of plot structure and writing techniques to the point that writers block would just be another part of the plot.
Around 2019 It felt almost finished, so I started to work on the 80'ish page prologue to set up some of the rules of the world and realized it had become almost vaguely educational. So in my infinite wisdom I decided to make it educational ... period ...
Now I can at least say with some confidence that I have pretty much everything planned out in a way that it can evolve naturally. A webtoon, a youtube animated series, a possible live action movie akin to last action hero or roger rabbit ...
I know this post will never get seen by more than the usual 3 to 4 people who like scrolling through this forum on a daily basis ... but that's alright with me. It doesn't need to be popular ... it just needs to be good enough for me.
At the time of me writing this there are 26 episodes up on webtoon.
Have a read in case you actually made it this far into my comment.