There's a long-running school of thought on the web that surfers make snap decisions about a web site. In The Big Red Fez, I said three seconds. No, said others. They said sites need to be deliberate and dense.
I was wrong. They were wronger. AJ points us to: news @ nature.com. Web users judge sites in the blink of an eye. Potential readers can make snap decisions in just 50 milliseconds.