A duration is the total amount of time required to complete a task, based on a task's start and finish dates. A duration is always based on how a day is defined in the working calendar associated with the task plan. A day is defined (by default) as being 8 hours; therefore, a duration of 1 day in a task is 8 hours.

A duration can be entered in minutes, hours, days, weeks, or months. However, durations (once entered) are always converted to days. For example, if a duration is entered as 4 hours, it will be be converted to 0.5 days in the task plan. You should try to use the same duration type (i.e. days) for all tasks in the same task plan.

A duration can be an estimated duration. When a duration is an estimated duration, a question mark (?) will appear alongside it in the task plan. For example, if a task play has a duration of 2 days?, that means that the duration is estimated at two days.

Durations are entered in two locations: from the Task Information dialog box or from the spreadsheet in the Gantt chart view. You can use abbreviations when manually entering durations in a task plan. Use a question mark (?) to indicate an estimated duration:

Duration Abbreviation
Minutes m, min, or mins. For example, 60m, 60min, or 60mins.
Hours h, hr, hrs, or hour. For example, 2h, 2hr, 2hrs, or 2 hour.
Days d, dy, or day. For example, 1d, 1dy, or 1day.
Weeks w, wk, or week. For example, 3w, 3wk, or 3week.
Months mo, mons, or month. For example, 2mo, 2mons, or 2month.

The amount of time to associate with a specific duration is relative. It can be based on previous experience with a similar task, based on the quality of the resource (or resources) that are assigned to the task, or it can be based on some other factor, such as an external estimate (for a contractor) or a fixed estimate (for a material resource). The important thing is to ensure that durations entered into a task plan are as accurate as possible. Any duration that is entered into a task plan should be flagged as an estimated duration until you are sure of the actual duration needed for that task in the task plan.

When you are creating the initial outline of a task plan, it can be helpful to assign each task a duration (preferably, an estimated duration), add dependencies (predecessors and successors), but not a constraint. Adding a constraint too early in the development of a task plan can make it more difficult to identify the basic schedule for the task plan. Once you have a good idea of the basic schedule in the task plan, you can then go in and fine-tune durations, dependencies, and add constraints (as needed).

As you progress with work on tasks you should update durations as more accurate information about those durations is available. When a duration in a task plan is updated, all calculations related to that duration are immediately updated.