How long should you stay in a career to make the time you spent getting the education worth it?

If you know that you want to change careers in the future, but you have only been in your current one for a few years, when is a good time to change? Especially, when you got a masters degree for your current career?How long should you stay in a career to make the time you spent getting the education worth it?
As long as you are enjoying and feeling fulfilled in your career chocie. That is and individual question that changes for everyone. No one can make a choice like that for someone else. if you are happy, then why change? Would you change because everyone else is or because it is now the %26quot;cool%26quot; thing to do. The trend in life now is to have 3 different careers, so keep up with the Robinsons? There is no set rule that says you ever have to change careers. That's because, you don't. It is all up to you and your want.