Automatic Updates will be sent to Contacts with a matching Match Profile when you publish your Listing, as long as the Make available for automatic updates option has been ticked in the Publish Settings on a listing record.
When a Match Campaign is started, it'll continue to send until everyone that was to receive the Listing has had their notification sent. So, if you have a Match Profile on a Contact that's set to receive a notification once a month, the campaign will be active until their notification is sent.
Only one campaign can be active at a time, so if you start a new campaign on the Listing, like a partial update or a price change, it'll cancel the part of the campaign that hasn't yet been sent.
Please Note: If Make available for automatic updates was not selected at the time of publishing and you turn it on later, you'll need to send a full update to send the Listing details to all matching Match Profiles.