Having played with longer times on wet tumbling, the brass doesn't start to rarnish, what happens is the soap suds run out. If enough soap to keep the gunk in solution it's fine. Definitely NOT necessary though to run lon time
If your brass is that dirty rinse it after an hour and let.it work with clean water again.
If your brass is that dirty rinse it after an hour and let.it work with clean water again.