There is also no default "can purchase weapons again" date for state (and federal) offenses.
I was under the impression no free man shall ever be debarred the use of arms.
In certain states the only way to own weapons legally is to get the governor of the state to explicitly pardon you, this is a terrible system, as is the case in states such as Illinois.
It makes sense to have a federal system for rewarding good behavior with the restoration of "rights", because the federal NICS is what would prevent these "second-class citizens" from buying weapons, in most cases.
Keep in mind, a simple kitchen knife counts as a weapon, and even people who have made mistakes have a right to life.
If you want to keep punishing them do it in prison.