In all versions of Project, in a task line, click a Duration cell to activate the spin button control and then click the arrows to increase or decrease the duration value in the cell. Each click changes the duration by one day. The Duration cell initially displays “1 day?” for a new task entry by default. The question mark sign (estimated duration label, “?”) in the cell disappears by the first click of any arrow.

Ismet Kocaman duration cell