Activity duration is a noun that represents the specific quantity of time that exists between the designated start time and end time of a scheduled activity. For further elaboration on this definition, please see the definitions for actual duration, original duration, and remaining duration. Activity duration should be carefully measured from the start date to the end date and can be calculated in whatever calendar units are appropriate given the anticipated timeframe of the project, such as days, weeks, months, etc. The activity duration can be established when a project is conceived, or it can be established when a project is kicked off, and is typically fluid in nature, and able to be modified if the situation merits and with the agreement of all parties. Furthermore, the sum total of the activity duration can also be broken up into smaller parts in the establishment of project milestones, points at which either the current state of the project will be assessed, or potentially sum or all deliverables will be submitted in either final or draft form.
This term is defined in the 3rd and the 4th edition of the PMBOK.