If the Web page survives this first — extremely harsh — 10-second judgment, users will look around a bit. However, they’re still highly likely to leave during the subsequent 20 seconds of their visit. Only after people have stayed on a page for about 30 seconds does the curve become relatively flat. People continue to leave every second, but at a much slower rate than during the first 30 seconds.

So, if you can convince users to stay on your page for half a minute, there’s a fair chance that they’ll stay much longer — often 2 minutes or more, which is an eternity on the Web.
Jakob Nielsen’s Alertbox