Why measure it
Response time is the single easiest thing to improve in most small businesses, and the thing most have never measured. An automatic email does not count. What matters is how long it takes for a real person to make contact.
How to use it
Log every new enquiry for one normal week. Note when it arrived, when a person first responded and when a proper conversation was booked. At the end of the week, the summary sheet shows your averages and where the longest waits happen.
Prefer a spreadsheet? Download the spreadsheet version, which works out your averages by channel as you go.