I think it's when you are doing it for the love, and it's also successful. The convergence of pleasure and productivity (in an economic sense).

By this logic, as a founder you may realize you've created something successful and fun but it's time to bring in different people to do things that you are no longer economically optimal to do.

Jeff Bezos is a great example of someone who clearly has a fun time every single day and whose creativity is still vital to Amazon's performance.