How to send escalation emails to an agent who has not responded on a ticket for a definitive period of time?

You can configure SLA policies such that there’s an escalation email triggered to the Assigned Agent before an ‘x’ period of time if there has been no response on the ticket. To do this, navigate to Admin-->Service Management --> Service desk settings -->SLA and OLA Policies and create a new SLA policy or edit the existing one and you would have the option for the same.