Activity duration estimating represents the act of quantifying the amount of time that it is anticipated the activity will take to complete. This phase of the project, that which consists of the estimating of the amount of time needed to complete all individual schedule activities, typically and traditionally takes place before a project is kicked off, during the conception phase, however, it is possible for the actual activity duration estimating period to take place later, perhaps close to or even slightly after the project has officially kicked off, however, even in those cases a draft or preliminary estimation has typically been made. Estimations can be made in any calendar unit that seems appropriate, such as months, weeks, days, etc., the entirety of the activity duration estimate can be further broken down into subparts or milestones at which certain elements, or deliverables, of the activity are to have been completed in final or draft form.
This term is defined in the 3rd edition of the PMBOK but not in the 4th.