Early Game Access* will begin on December 13th at 7:00AM EST, 2011 and ends on December 19th, 2011 at 10:01PM EST
That suggests you won't be able to log in without a code from 19th at 10:01pm EST.
Your created characters will stay for afew days at least though I havn't seen any official confirmation of how long. I should think a couple of weeks would be safe. But thats a complete guess on my part.