Do something new every three years:
I was thinking about the three-year rule while reading about Malcolm Gladwell’s observation that it takes 10,000 hours to become truly expert at something. If you really throw yourself into a job, you’ll spend 60 hours a week working. That’s 3,000 hours a year (allowing for vacation), which means you’ll hit the 10,000 hour mark a few months after your third year.
So maybe that’s where the three-year rule comes from. You’re now expert at what you set out to master. Great. Now go do something else.
Great idea! The article also reveals some of the inner workings of The Economist. A highly recommended, quick read.
Adam Keys is typing 
Meaning, you should quit right before becoming an expert?
The way I look at it, you should always challenge yourself. As you reach the expert level in one skill, you should throw yourself at something completely different. For instance, I’m trying to improve my ability to do visualization and interaction design work so I can go from dilettante to confidence in actually doing it.