According to a 3-year-old study that I just saw, the average length of time it takes someone to decide a web page sucks is about 50 milliseconds.
To put this amount of time in perspective, this is 3-4 times faster than your fastest muscle reflex.
The most important elements of a landing page, to avoid the 50 millisecond puke, are header and page design.
The implications go far beyond the first 50 milliseconds. From the article:
If people believe a website looks good, then this positive quality will spread to other areas, such as the website's content.
Since people like to be right, they will continue to use the website that made a good first impression, as this will further confirm that their initial decision was a good one.
Is your website making a good first impression?