First create fields rent_duration_unity in your own module then create class and use this rent_duration_unity fields which you created first in .py file, as shown in code given below.

 'rent_duration_unity' : fields.many2one('product.uom', string='Unity', 
                                 help='The duration unity, available choices depends of your company configuration.')

Now use this function in relate to fields in your .py file in openerp, the function is given below

def default_duration_unity(self, cr, uid, context=None):

        """
        Returns the 1st UoM present into the Duration category.
        """
        unity1 = self.pool['product.uom'].search(cr, uid, [])[1]
        unity = self.pool['product.uom'].browse(cr, uid, unity1)
        if not unity:
            _logger.warning("It seems that there isn't a reference unity in the 'Duration' UoM category. "
                            "Please check that the category exists, and there's a reference unity.")

        return unity.id if unity else False