Definitely should be configurable - it was just an example.   

I use 30 seconds here (e.g. deliberately long) as most clients time out waiting for a banner between 30-60 seconds, so 30 seconds was as long as I could make it so that most clients see the tempfail and don't time out and drop the connection before this.   You wouldn't want to do this if you were talking to an MUA though (so bypassing any wait if connection.relaying is also a good idea if this plugin runs after any acl plugins).