I'm not sure I can answer with as much precision as you'd like. There are rational considerations, of course-- I'd want to make certain I'd be financially secure, that, if I were still working, I would be able to comfortably divide my time between pursuits. But the emotional considerations are just as important, and harder to quantify.

I'm not sure there is an optimal time, necessarily. Right now my career satisfies me, in terms of planning for the future. At some point it might not. I can't say when that will occur, only that it hasn't yet.