The Duration field is populated when you import data from MS Project, and it displays the amount of time in days that it will take to complete the current task. A day is 24 hours. So for example, 1 day is 24 hours, and .33 days would be 8 hours.
A day is considered to be 24 hours even if you have a Project Calendar set within MS Project with an 8-hour working day. The conversion to a 24-hour day is done when you import the data from MS Project.