I'll echo Vinnie's post by noting that it's about 15 miles (all freeway) from the JerryDome to the South Entrance for DFW Airport.
Once you're into the airport, you'll need at least 15 to 20 minutes just to get parked and into your terminal.
DFW Airport is physically the biggest airport in the USA if I'm not mistaken. If you've not been there before you'll want to allow the needed minutes to make sure you're getting off at correct terminal (I think there's eight), parked etc