Find it.

I use a functional field "duration_in_day" whose the value is equal to the duration field's / 24.

Here is my code :

class crm_meeting(osv.osv): 
    def compute_duration_in_day(self, cr, uid, ids, field_name, field_value, arg, context=None):
        records = self.browse(cr, uid, ids)
        result = {}
        for r in records:
            flt = 0.0
            if r.duration: flt = r.duration / 24
            result[r.id] = flt
        return result

    _inherit = 'crm.meeting'
    _name = 'crm.meeting'
    _columns = {
        'duration_in_day': fields.function(compute_duration_in_day, type='float', string='Durée en jours'),
    }

crm_meeting()

However i don't think it's optimised.