That depends on the individual. You will be learning a new skill.

I have seen people solo after as little as 3 lessons/sessions with an instructor, then again I know of a couple of people that still haven't soled after 18 months of being at the field at least twice a week.

Becoming "good", like most things, takes practice.

Also, do you mean "good" as the ability to successfully fly an aerobatic plane ? "Good" as the ability to fly a competition type pattern, or "good" as the ability to successfully compete against someone like Chip Hyde ?

If you mean good as the ability to successfully compete against someone like Chip Hyde, you are probably talking years.