Time yourself for a fortnight before trusting any number. Almost everybody underestimates, because the figure in your head is the time with your hands in somebody's hair and not the appointment. The appointment includes greeting, consultation, mixing, cleaning down and the client putting their coat on.
Book the honest duration, not the fastest one you have ever managed. A schedule built on your best day runs late every other day, and running late is the thing clients complain about most and forgive least.
Where a service genuinely varies — length, thickness, removal of somebody else's work — use the longer duration and finish early rather than the shorter one and overrun. Finishing early is a gift; overrunning is a domino that lands on everybody booked after them.
Watch for the services that always overrun. Two or three of those in a week are not bad luck, they are a duration that is wrong, and the fix is one number rather than a faster pair of hands.
Duration also drives price. If a service takes half again as long as you booked it for, you are not just running late, you are working under your hourly floor every time somebody books it.