The guys I know personally who are successful, who have become "overnight" successes recently (like box.net, airbnb, dropbox, and so on) have been at it (on the same company, not to mention the priors) for 4+ years beforehand or longer.

Wow, 18 years, I guess I still haven't learned my lessons!

Just to be clear, I'm not disparaging anyone who has experienced success quickly, but companies like Dropbox are an aberration, even if they weren't an "overnight success". The more realistic company probably takes a few years to hit 10 employees, and a decade or more to hit 100.