Customer says that they estimate about 80,000 Registration list items in this list per year, so obviously some size management is necessary. They need to create some type of archive with condition "old list items after they had been completed for 90 days".
As an option Deleted Registration list could be used for this purpose. When you delete Registration items according to condition then item is placed to Deleted Registration list.
You could find it out Site actions->View All site content->at Lists deleted Registrations.
Also you could export to excel from Registrations list as result you will have external storage of Registrations information.