DateTime start = new DateTime(2010, 8, 25, 8, 0, 0);
DateTime end = new DateTime(2010, 8, 25, 16, 0, 0);
Console.WriteLine((end - start).TotalHours);
для строк:
DateTime start = DateTime.Parse("8:00");
DateTime end = DateTime.Parse("16:00");
Console.WriteLine((end - start).TotalHours);