To clarify: Before this question, from a pretty much "stock"
configuration, I was getting the symptoms shown above. After this
question, and some tinkering, I have gotten the time down somehow to
disconnect minutes after LOGOUT, ETC.; without the need to resort to
the solutions below.
The best, trustworthy, answer I can find is from O'Reilly Online.
This initiates a periodic (four-hour) check to see if the client has disappeared. Expired connections are addressed somewhat better with Samba's keepalive and dead time options. All three eventually arrange to close dead connections, returning unused memory and process-table entries to the operating system.
So basically, it is closed after just 4 hours! But as also said, it's best to use Samba's keepalive option to close this far gap. Of course, even then, since Samba's/Windows's/Etc. ability to close and reopen said ports is of no problem, I will resort to using deadtime instead to just close the unused gap; whether or not the client is still there.