There is a current configuration to allow users to choose to remember a device for MFA: https://docs.microsoft.com/en-us/azure/multi-factor-authentication/multi-factor-authentication-whats-next#remember-multi-factor-authentication-for-trusted-devices
Additionally, we’re looking at providing more controls for this through conditional access.
We have started working on this feature and hope to have another update by Oct 2018.
We continue evaluating several alternatives to provide full email customization. We are actively working on an alternative.
Unfortunately we do no yet have an ETA.Chris Webb commented
it would also be good to be able to configure the expiry time in custom policies, as some mail providers/forwarders are poor, and messages not delivered to users' mailboxes before the 5 minutes has already passed