There isn't really a set date for when you've officially improved-and-therefore-worthy-of-this-meme, surely. You can improve in 3 months, and you can also improve in 3 years.

How drastic the change is might be linked to the amount of time spent working on it, but the meme states "take old piece, redraw, look at improvement" so presumably the artist can notice the differences over any span of time. There might be large differences and there might be small, subtle differences in the redrawn picture, and both are just as valid as the other.