You can use days or hours in the Gantt chart. The easiest way of setting your duration and your preferred measure is to type the duration followed by the measure shorthand into the Duration field of the Gantt grid
- Ensure your map is in Gantt view
- Click within the "Duration" field of the Gantt grid for a task
- Highlight the current duration
- Type for example "4h"
- Hit the tab key to commit the new value
Notice that the Gantt grid accepts the "h" shorthand and interprets it to mean "4 hours". Likewise, for a task that you wanted to measure in days you could enter "2d" and the task would be scheduled to last 2 days.
You may find it helpful to change the timescale used if your tasks are measured in hours. You can change that via the "Timeline" menu on the contextual "Gantt" ribbon.