How is this flow being triggered? Is it part of the approval flow or separate? It would need someway of being triggered.
One method could be to setup a separate "Reminder" flow that runs using the "Schedule" every "x" amount of "time". When that trigger is hit, it cycles through all requests in the SQL database and compares the current date/time to the stored date/time in SQL. If it hits the condition, it pulls the email from SQL and sends a formatted email.
For the Manager vs Head, if you are using 2 different status', another conditional to check the status to know if it should pull the manager or head email address.