The grace period specifies how long the mailbox can be over the quota (disk space or number of messages) before messages are bounced back to sender. The grace period is not how long the message is held in the message queue, it’s how long the mailbox is over quota before all incoming messages, including those in the message queue, are bounced. (see 20.1 Overview for more details.) The grace period starts when the user has reached the quota threshold and been warned. See To Setup Quota Notification.
To specify a quota grace period at the command line:
configutil -o store.quotagraceperiod -v number
where number indicates number of hours.