How fast do manual changes session_config.vars (through the Room admin panel) propagate if I were to change it while an online session is ongoing?

I am working with a vendor that does not react immediately in terms of shutting down external invitation links, so I am thinking of setting a session parameter 'shudown'. Then I set it to zero when I want just to redirect subjects to a "Survey Closed" page (instead of removing everything and giving a HTTP error).

You can set session vars through the REST API.

But oTree admin interface doesn't let you modify session.config or session.vars of an already created session.

And the changes apply immediately.