I don't think that's right... The FAA allows a max duty day of 16 hours for part 121 operators (air carriers). So, I would expect DAL's max duty day to be somewhere slightly less than the FAA's limit.

Air Canada, for example, has a max duty day of 14 hours (and like the FAA, the max duty time allowed by Transport Canada is 16 hours). That comes in helpful for longer trips, but pretty much anything international would require an overnight stay. Crews are also required to get a minimum of 8 hours rest so maybe that's where the 8 hours comes from. Anything less than 8 hours and you're considered still on duty -- this is called a Continuous Duty Overnight, or CDO. Doesn't sound like fun, eh?