The due date (due) overrides duration. If you specify both duration and due, the command ignores duration.
Specify the duration in the ISO 8601 format. For example:
P1Y2M3DT1H30M10S represents a duration of 1 year, 2 months, 3 days, 1 hour, 30 minutes, 10 seconds
PT1H30M represents a duration of 1 hour, 30 minutes
P1D represents a duration 1 day
PT15M represents a duration of 15 minutes
Notice that the T in the string separates the date information (year, month, day) from the time information (hour, minute, second).