30 Days, 60 days, 6 months and 1 year after submission I send out update emails which let the user know the rank of their website. If the email bounces back to me, I set a "noshow" flag against the users website(s) which disables them in Craigs Directory (please see Where's My Website).
I have broken the results down into groups of 10,000 website listings. I'll let the graphs do the talking (basically, the red bit is the percentage that have bounced).
Websites 1 - 10,000
Websites 1 - 20,000
Websites 1 - 30,000
Websites 1 - 40,000
Websites 1 - 50,000
Websites 1 - 60,000
All Websites (as of 18-Feb-2011)
PS. Website approvals are now taking 2 - 3 months so I have changed my update email strategy to 6 months, 9 months, 1 year and 18 months. This could make matters worse.