In Structure.Gantt, duration for each task can be calculated in one of three ways.

Automatically

When tasks are automatically scheduled or manually scheduled without a fixed duration, their duration is calculated automatically, based on their work estimates and the Work Estimates settings associated with that task.

To adjust the duration of these tasks:

Dragging Automatically Scheduled Tasks

If you drag the right side of a task that is automatically scheduled, the task remains in automatic scheduling mode, but its duration is updated. If you drag the left side of the task, the task becomes manually scheduled based on the new Start Date and it's duration is updated accordingly.

In either case, the duration will be updated as follows:

Dragging Manually Scheduled Tasks

When you drag the side of a manually scheduled task, if the side of the task you are dragging was manually scheduled, the corresponding field (Start or Finish Date) will be updated to the new date and the duration will be updated accordingly. Otherwise, only the duration will be updated.

In either case, the duration will be updated as follows:

Based on Start and Finish Dates

When tasks are manually scheduled and have both a manual Start and Finish Date, the task has a fixed duration, spanning the time between its Start Date and Finish Date.

To adjust the duration of a fixed duration task, drag either side of the task bar. This will update the task's Start or Finish Date, and the duration will be adjusted accordingly.

Adjusting the duration of a fixed duration task does not affect its work estimate.

Based on Sprints

When tasks are scheduled based on sprints, they have a fixed duration corresponding to the length of the sprint. It is not possible to adjust the duration of these tasks, except by removing them from sprint-based scheduling or changing your sprint duration settings.