It can take 24-48 hours for CMS' RDS Center to process a retiree list. For the purposes of submitting a timely RDS application, the Plan Sponsor is simply responsible for submitting the file by the Application Deadline. The file does not need to be processed by the deadline.
Note that it can take several days for the Initial Retiree List Received Date to display on the Application Overview page after a successful file upload.