When is the right time to move on?

I would like to follow up my previous post about the reason of working. I have come to the point when I have started to wonder when do we actually realize the point, that it has been enough and move on to another company/place… How can we measure the marginal learning(and maybe the curiosity of change)?

I have decided to move on and to go for a change, however I am very happy at my current work.
Reason? Change, new things, new encounter, new learning, new challenges and my inner feeling. There is no time to be waisted. I was reading a book by R. Kyiosaki and he said, that young people should change job on regular basis, not to stick with one company for many years. I think, that it depends on field we are working and on our aim. If we want to specialize and make a one-company carreer(climb the ladder), no wonder if we stay 1++ years. But for general learning change is fine.

BUT, this kind of change requires a strong decision and determination. And vision - knowing what you want and how to get there.