There have been a few posts here from folks who have called Pentax and they have said within 90 days or so of the purchase of the camera.
On the other hand, since the extended warranty is essentially just a post card with a serial number printed on it, so it not like they need to go down to the beach, get the sand and melt it, to make the glass. Not a large and complex task to produce them. Also, since they are a product of Pentax USA, its not like they need to go to Japan to get them printed and then ship them over on the "slow boat".