Answer

The default setting for deletion for all SWD packages on the local client machine is 7 days.  If the Package is deleted on the server and clients still have the packages, the events generated from the packages will cause ItemNotFoundException errors.

The 7 days begins from the time the client no longer sees the package in the Client Configuration XML sent from the server.

As such, this is the best practices for retiring/deleting packages:
1. Disable all tasks associated with the package (if a task is enabled, and a client has that task, the 7 days will not start until that task is gone or removed).
2. Tasks can be deleted very quickly since an updated Client policy XML will stop any events for that task from being generated.
3. 7 days after the tasks were disabled/deleted, delete the package.