After your wall has ended, we do not delete any of the stats or posts. They'll remain in your account forever so that you can compare the performance of future walls to this event's wall.
Approximately 30 days after your event has ended we'll automatically do a process that we call archiving. In this process, we take all of your posts that we pulled into your wall (in all queues) and then archive them into a special format so they are no longer in our primary database systems, and because of this we also then "lock" your wall so that you can no longer move posts around in the queues. The reason we do this is so that our systems can remain speedy for our other customers, since in most cases 30 days after the event has ended there is no longer a need to move posts around in their queues.