What is an estimate of activity duration?
An activity duration estimate is an evaluation of how long will be needed to actually perform an activity. The estimate isn't how long you want the activity to take, or how long someone tells you it must take; the estimate is how long the person doing the work really thinks it will take.
If people believe duration estimates are unrealistic, they stop trying to achieve them and when delays occur during an activity, people accept them as inevitable instead of seeking ways to overcome them.
Note that duration is different from effort. Effort represents how many hours a person will actually be working on an activity. A person expecting to spend a total of 8 hours of effort at the rate of 2 hours each day would estimate an activity duration of 4 business days.