When to sell: It's the age-old investing question. Berkshire Hathaway (NYSE: BRK-B) CEO Warren Buffett says his favorite time frame for holding a stock is "forever." Investing great Peter Lynch says you should sell a stock when your reasons for buying no longer apply. And legendary fund manager John Neff says it's "probably time to sell when you feel like bragging."  

Motley Fool co-founder David Gardner has racked up some big returns for investors by recommending stocks such as Amazon.com (Nasdaq: AMZN)Intuitive Surgical (Nasdaq: ISRG), Marvel (Nasdaq: MVL), Netflix (Nasdaq: NFLX), NYSE Euronext (NYSE: NYX), and Take-Two Interactive (Nasdaq: TTWO). At a recent Washington, D.C.-area Motley Fool community event, David shared his thoughts on when to sell.

Amazon, Berkshire, Marvel, and Netflix are Motley Fool Stock Advisor recommendations. Berkshire Hathaway is also an Inside Value pick. Take-Two, NYSE Euronext, and Intuitive Surgical are Motley Fool Rule Breakers recommendations. Read more on David's stock picks with a free 30-day trial subscription to The Motley Fool's investment services.

The Fool owns shares of Berkshire. David owns shares of Amazon, Intuitive Surgical, Marvel, and Netflix. Mac doesn't own any of the stocks discussed. The Motley Fool has a disclosure policy.