Comscore recently published a press release about 1st and 3rd party cookie deletion rates that they measured in the month of December.
Here are the important details.
Duration: December 2006
Sample Size: 400,000
Sample Site: Portal with content and an Ad serving 3rd party cookie
Average number of 1st party cookies per computer measured: 2.5 (100 visitors becomes 250 visitors)
So, [...]