Duration is the slot
A service's duration is the block of time it takes on your calendar. Available start times are offered inside your working hours so the whole service fits before you close.
Include your real work
Set the duration to include setup, cleanup and notes. If a 45-minute service really needs an hour, book an hour — otherwise you run late every day.
Different lengths of the same thing
When a service genuinely takes different lengths of time, create separate services (for example a short version and a full version) with their own duration and price. Customers then choose the one they need and your calendar stays accurate.