Outlook metric

Hi, I hope you can answer this question. I am working with an outlook database, and to know the days elapsed since an email with a case to be resolved is received until its resolution, but I would not know how to put together the formula (I have the dates of sent and received), since there would be many mails (different cases per client) I thought of doing it by subject: when it starts, put region_customername_test1 and when region_customername_resolved_test1 is resolved. Would you have any suggestion? Is this possible to do? Thank you in advance