BusinessHours bh = [SELECT Id, Name, IsActive, IsDefault, SundayStartTime, SundayEndTime, 
                    MondayStartTime, MondayEndTime, TuesdayStartTime, TuesdayEndTime, 
                    WednesdayStartTime, WednesdayEndTime, ThursdayStartTime, ThursdayEndTime,
                    FridayStartTime, FridayEndTime, SaturdayStartTime, SaturdayEndTime,
                    TimeZoneSidKey FROM BusinessHours WHERE IsDefault = true];

DateTime startTime;
DateTime endTime;
Datetime dt = System.now();
String dayOfWeek = dt.format('EEEE');

if(dayOfWeek.equalsIgnoreCase('Saunday')){
    startTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.SundayStartTime.hour(), bh.SundayStartTime.minute(), bh.SundayStartTime.second(), 0));
    endTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.SundayEndTime.hour(), bh.SundayEndTime.minute(), bh.SundayEndTime.second(), 0));
}else if(dayOfWeek.equalsIgnoreCase('Monday')){
    startTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.MondayStartTime.hour(), bh.MondayStartTime.minute(), bh.MondayStartTime.second(), 0));
    endTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.MondayEndTime.hour(), bh.MondayEndTime.minute(), bh.MondayEndTime.second(), 0));
}else if(dayOfWeek.equalsIgnoreCase('TuesDay')){
    startTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.TuesdayStartTime.hour(), bh.TuesdayStartTime.minute(), bh.TuesdayStartTime.second(), 0));
    endTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.TuesdayEndTime.hour(), bh.TuesdayEndTime.minute(), bh.TuesdayEndTime.second(), 0));
}else if(dayOfWeek.equalsIgnoreCase('Wednesday')){
    startTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.WednesdayStartTime.hour(), bh.WednesdayStartTime.minute(), bh.WednesdayStartTime.second(), 0));
    endTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.WednesdayEndTime.hour(), bh.WednesdayEndTime.minute(), bh.WednesdayEndTime.second(), 0));
}else if(dayOfWeek.equalsIgnoreCase('Thursday')){
    startTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.ThursdayStartTime.hour(), bh.ThursdayStartTime.minute(), bh.ThursdayStartTime.second(), 0));
    endTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.ThursdayEndTime.hour(), bh.ThursdayEndTime.minute(), bh.ThursdayEndTime.second(), 0));
}else if(dayOfWeek.equalsIgnoreCase('Friday')){
    startTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.FridayStartTime.hour(), bh.FridayStartTime.minute(), bh.FridayStartTime.second(), 0));
    endTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.FridayEndTime.hour(), bh.FridayEndTime.minute(), bh.FridayEndTime.second(), 0));
}else if(dayOfWeek.equalsIgnoreCase('Saturday')){
    startTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.SaturdayStartTime.hour(), bh.SaturdayStartTime.minute(), bh.SaturdayStartTime.second(), 0));
    endTime = DateTime.newInstance(Date.today(), Time.newInstance(bh.SaturdayEndTime.hour(), bh.SaturdayEndTime.minute(), bh.SaturdayEndTime.second(), 0));
}

//get the difference between start time and end time
Long difference = BusinessHours.diff(bh.id, starttime, endtime);
Double hours = difference/3600000;
System.debug('Business Hours Diff-' + hours);