Answer:
The user can continue to view the page they are currently on, but if they navigate away from the page, they will be logged out.
Step-by-step explanation:
At 7 PM, the user will be effectively logged out of the system but the user will still have access to his current page. Having being logged out of the system, the have the access to perform any DML, read write, execute operation and at the same time the user won't be able to open any new page; navigating away from this page, the user will be notified that they're logged out.
This is possible because the moment the user sends a new request to the server to execute some DMLs or to navigate, the system logs out the user immediately signaling a session expiry .
Conclusively, already logged in users will only have Read-Only access once mode once the Login Hours has elapsed for the organization. No consequent saves or edits can be made to the database, at this point.