"There was also a popular article(NYT?), which I can't find the link to at the moment, that showed me that only if you invest in the market for around 30 years or more, can you get a decent return. To me that makes sense if you want to leave your kids a little something after you're gone but not for your own lifetime."
What about investing for your own retirement? If you start investing when you begin your first job (usually in your 20s), and you expect to retire in your 60s and live into your 80s, that'll certainly give you at least a 30-year investment horizon.