Google published a list of key dates related to the hosted domain product retirement within AdSense. Starting this week (February 22), AdSense publishers will no longer be able to start using the product, and by March 21, existing publishers will no longer be able to create new hosted domains. By April 18, hosted domains will become inactive, and the service will be removed entirely from AdSense accounts by June 27.
A guide to migrating domains to new domain parking providers was put together by Google and is available here.