It is usually measured per email and reported as a median or an average over a week. Some teams count only business hours, others the clock time the customer experiences; both are fine as long as you stay consistent.
Why it matters
Customers judge speed by the first reply, not the last. A quick, useful first reply often ends the conversation; a slow one produces a second email (“any update?”) that adds to the pile. For small teams the goal is not instant replies but a predictable window, such as the same working day, that customers can count on.
First response time says nothing about quality on its own. A fast “we’ll look into it” followed by silence scores well and helps nobody, so read it next to how many emails needed a follow-up.
An example
A customer writes at 9:10 and gets an answer at 11:40. The first response time for that email is two and a half hours, or less if your team only counts from opening time.
What to aim for: response times for a small team.