When you add a new task to a workspace, as a default it will be allocated a 1 day duration. To change the duration, find the task in the list or on the Gantt chart and click on its title.
In the task's detail panel click 'Due date'. To add a duration to your task, select your start date, the next date you select will become the task's end date. The duration will be calculated automatically.
If you have selected 'Automatic work-day correction' in the task workspace options, the end date will be adjusted accordingly.
You can also adjust the duration of a task on the Gantt chart by clicking and dragging either end of a task’s bar; unless you have “Automatic work-day correction” selected.