Jakob Nielsen: Time Lag Between Visiting and Buying

Users often convert to buyers long after their initial visit to a website. A full 5% of orders occur more than four weeks after users click on search engine ads. However research shows that 75% of the conversions occurred within 24 hours. Research was done by Dr. Alan Rimm-Kaufman from the Rimm-Kaufman Group, they tracked one million clicks on search ads on Google and Yahoo. These advertising clicks eventually translated into 41,377 conversions on the target websites. via useit