If a member is assigned a Level that is set to Expire and they are viewing a protected page or post and then their Level expires, they could still be able to view that protected content until they navigate away from that page/post or if the page/post is refreshed in the browser.
So, they would retain access if they don't navigate away or refresh the page/post after their Level expires.
This is based on how web pages work (if you stay on a web page and don't refresh, you can continue to view it).
Typically, a user will navigate away from pages as they check out the site and this won't be an issue in most cases. But, if content is protected and only available for a short time (Example: set to expire after 1 day) then this is something to consider.
A forced page refresh can help with this.
If the page refreshes after the Level expires, they wouldn't be able to view the protected content any more.
There is a plugin in the WordPress Plugins directory named Force Refresh.
This plugin (or one like it) could be used to set a refresh on the site or a per page/post basis and set the refresh intervals (timing).
You might want to consider setting the refresh for 3-4 hours (whatever works best for you and your members)