Workflow items can't have duration associated with them, but you can use the duration based grids to visually give the task a duration based on the grid. In Beta 1, the grid allows you to have a number of durations including days, hours, months, or years. The workflow items are then resized to fit within the grid as desired.
Unfortunately, the item cannot be shrunk enough to fit exactly within a single day when the duration based grid is set to days. We are addressing this in Beta 2.
Principal Product Manager