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.

Also, some inside baseball on The Economist therein. Great idea, quick read.