Choosing the number of hours
An hourly duty works best when the stops are close together and the schedule is predictable. Count the travel time between stops as well as the time at each one, because the booked window covers both.
If your plan could overrun, tell the team up front so the duty can be discussed with a realistic end time instead of being extended repeatedly on the day.
When the stops add up to most of a day, a full-day duty is usually a cleaner arrangement. The team can help you choose after you share the plan.