When should I return to receive my one-year permission to stay after my retirement visa application in Thailand?

Process/timing question. On 30 November I applied at CM (airport office) to convert my 45 day tourist to a retirement/permission to stay. They folded up and stapled a blue form inside my PP (I believe it was an “under consideration” form) with instructions to return 23 December. I assume this is to receive my 90 day stamp? If all of the above is correct, my question is when can I go back (with all of the appropriate forms) and get the final stamp indicating I am now on a one year permission to stay? I have heard immediately as well as when I am within X days of the expiration of the 90 days. Thanks.