you can use float_time widget, in combination with fields.float

add the field like this:

'duration': fields.float('Duration (in hours)'),

AND ALSO ADD THE FIELD IN VIEW

<field name="duration" widget="float_time"/>

it is been answered on stackoverflow, I don't know whether we can Share the Links of other sites here, so I just provided the code I know and Also I have found in the trusted resources.