Hard to say. Spent my entire life building up to it, so I'll put it into a very rough timeline instead:
I'm 27 now and have been posting creative projects/making actual content ever since I was about 16, with varying degrees of success. (I'd already been writing and designing characters since age 6~7 or so.) Dabbled in a ton of different things and eventually became a full-time creator 2 years ago.
For comic work specifically, I started posting my first one in late 2015 and managed to gather an audience, but had to take a break from it starting 2017 (after which I did completely unrelated creative stuff out of necessity) and only returned to it (via a complete remake) in October of last year. It's been going well but focusing on it full-time is a risky move on my end based on nothing but my belief that I'm onto something good. (If there's one thing I've learned over the years, it's how to analyze my own failures, so I'm pretty confident on this one - still, stability is...something else lol)
I think my main takeaway from my own experience is that you'll always want multiple income sources and just keep getting to know people - I earn pretty much nothing on the platform here itself, but you never know who'll become a patron and/or die-hard fan who commissions you to draw your own characters haha.