The obvious is "poker isn't solved, yet", but in the spirit of the question, about 10 years.
It took about that long before I stopped feeling like I was learning new conceptual ideas and was instead making refinements to my existing game.
My progress is marked only in retrospect and the large turning points are marked by VPIP changes. I went from playing too many
hands, to playing too few, to playing too many, to balanced. I was certain I was balanced at each stage and didn't realize my mindset errors until I spent time thinking about where I was.
In the specific, I went from a fish playing everything, to a rock playing premium only, to over playing marginal draws and second best holdings, to balanced. The journey took about 10 years.