And had this reply this morning:
Thank you for contacting Thor Forums support desk. If, when logging in, the member checks the Remember Me box they won't get logged off. They also need to set their device to accept cookies, this also keeps a member logged in even after an extended period of inactivity.
I had assumed that Remember Me meant to automatically log me in when I went to the site. But apparently this does accomplish what I'm looking for. Unless you explicitly hit the Log Out button you will never be logged out.