IMO, the answer to OP from most people would be something that roughly translates as "whenever it starts paying my bills and giving me some measure of comfort while looking it won't stop anytime soon."
I mean, that's usually the chief reason to start any job, full-time writer included.
As for me, personally, I did something like
@Ace_Arriande, in 2016. I took 1-year vacations (my loving mom paid my bills) and focused on writing. I'd either make it or give up on the dream of being an author for good.
I got lucky the LitRPG genre was just starting and people were thirsty for books. I released chapters on RR and had a successful launch on Amazon. Releasing more books in the series pays my bills to this day.
In my experience, besides sheer luck (like in my case), the secret to getting however many numbers you are aiming for is consistency, and being serious about improving your craft. You gotta release chapters and you gotta try to do your best in each of them.
Research creative writing, read/watch successful authors talk about it. Brandon Sanderson's online creative writing classes are great btw.
And don't. Stop. Writing.
Creative types (me included) sometimes get annoyed at the boringness of grinding chapters. I mean, it's great to have an idea and put it on paper at first, but then comes the part that feels much like any other work, and we hate it.
Guess what, you must work to get paid. If there's a perfect, grind free job in the world, I didn't find it yet.