It is so hard to tell what is crap though - often there is such a sharp tipping point. I can't tell you how many times in my life the golden egg came from the very last email I sent out, etc. It's super common.

Sometimes I've quit when it was too early. In high school I was selling chinese mp3 players on a website. I did some marketing of various types and didn't really know seo yet. I eventually gave up because demand was low. (How could I compete against ebay, I thought?) Two years later, I checked analytics, which I didn't know about at the time. It turned out a month or two after I quit I got tons and tons of traffic but by then I had broken the navigation - I happened to have an article I wrote rank for "chipod" or something along those lines which became a popular term later.