JC,
As chains and sprockets are reasonably costly, you don't want to replace them at all if you can aviod it. Wd40 should never go near a chain for two reasons, firstly it degrades 'O' and "X' rings as Pig9r said and secondly Wd40 is penetrating oil and is likely to pass the rings and dilute the sealed in lubricant. Kerosine, which is much cheaper than WD40, is recommended by some chain manufacturers and does a great job of cleaning.
A chain which is well maintained should last forever on a bike doing 3k miles a year.
Check out this thread.....it has the topic well covered and has links to some good information.....
Chain Cleaning
[ This message was edited by: Legman on 2005-10-10 01:30 ]