Both. So, during the next check in interval the IP will get updated in the JSS. And of course it gets update at inventory time as well.
In our case it happens more or less immediately because we have a process that checks connection to the internal network whenever a network change occurs and sets firewall settings appropriately.
You could build something similar if you wanted immediate update of IP info. Basically a LaunchDaemon that would use /Library/Preferences/SystemConfiguration as a WatchPath and upon change, just do a 'sudo jamf log' That will be enough to get the new IP info back into the JSS.
