How can I use the date of the last email to the requester in a report?

How can I use the date of the last email to the requester in a report?

I want to create a report that lists all open requests that have not had an e-mail to the requester in X number of hours.  For instance, the report might show any request for which an email has not been sent to the requester in 48 hours.

We want to establish an internal SLA which states that no request will go more than 2 days without the requester receiving an update as to the status of their request and need this report to help us identify any requests not meeting that SLA.
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