Activity duration specifies the length of time (hours, days, weeks, months) that it takes to complete an activity.
This information is optional in the data entry of an activity.
Work flow (predecessor relationships) can be defined before durations are assigned.
Activities with zero durations are considered milestones (milestone value of 1 to 94) or hammocks (milestone value of 95 to 99).
The best estimate of the time necessary for the accomplishment of the work involved in an activity, considering the nature of the work and resources needed for it.