How should I report DM response time to a client?

Report the median time to first reply, not the average, and say whether it's measured in business hours or clock hours. Show high-priority messages, such as leads and complaints, separately, because that's the number the client cares about most.

The average is distorted by one message left over a weekend. Twenty replies inside an hour and one answered on Monday morning can average three hours, which describes none of them. The median stays under an hour.

Business hours versus clock hours matters just as much. A message at 11pm answered at 9:15am is a 15-minute reply in business hours and a ten-hour one on the clock. Agree which you'll use and don't switch.

Keep the client's 24-hour limit in mind too: Meta stops normal replies 24 hours after the person's last message, so anything slower than that may not have been delivered at all.

The full method is in What to Put in the Inbox Section of a Client Report (and the Numbers to Leave Out).