I have a trigger set up to send an email 5 days prior to arrival (at 7 am), to any guest who has not yet signed the rental agreement.
If they fail to sign the agreement, does checking this box mean that they will get the same email each day (at 7 am) prior to the arrival until it is signed? How often is the same email sent via this trigger?
No, each individual trigger will only send the email once. That checkbox is for situations where the guest might not meet the criteria at the appointed time, but may later. For instance, you might have an email containing the lock code that is sent only if the guest has signed the agreement - that would be an appropriate situation to use that checkbox, as the guest may not have signed the agreement today, but may sign it tomorrow, in which case they should get the code even though the normal time for it has passed.