I'd say when you are pushing your plane past what it's originally designed for and getting away with it because you are tired of the usual manuevers is when you are ready. Each person has a different learning curve, so can't just put a generic, after x amount of hours you are ready.

Personally, I like having a plane that's a challenge and can't go back to the easy to fly birds any more because they just don't give me any more excitement. I have a cub I picked up early on, kept it on the shelf for too long, I took it out on 3 flights, was bored stiff with it since I already moved way past it design wise before it's maiden, so there is something to be said about not being overly conservative.