best response confirmed by shehanjp (Contributor)



So, the following is from the Session Controls within a CA policy - "Time period before a user is asked to sign-in again when attempting to access a resource. The default setting is a rolling window of 90 days, i.e. users will be asked to re-authenticate on the first attempt to access a resource after being inactive on their machine for 90 days or longer".
So I would think that you can work on the principle that after 2 hours of inactivity, they will be prompted for sign in, and at this point, they will be challenged to register for MFA.