BusinessHours bh = [SELECT FridayEndTime,FridayStartTime,Id,MondayEndTime,MondayStartTime,
                    FROM BusinessHours where Name='Test Business Hour'
                    LIMIT 1];
Time startTime = bh.MondayStartTime;
Time endTime = bh.MondayEndTime;

Date d = Date.today();
Datetime monTodaystart = Datetime.newInstance(d.year(),d.month(),d.day(),startTime.hour(), startTime.minute(),startTime.second());
Datetime monTodayend = Datetime.newInstance(d.year(),d.month(),d.day(),endTime.hour(), endTime.minute(),endTime.second());
System.debug('monTodaystart :: '+monTodaystart);
System.debug('monTodayend :: '+monTodayend);

Long diff = BusinessHours.diff(bh.Id, monTodaystart, monTodayend);
System.debug('diff  :: '+diff);