Assuming there isn't anything going on at Texas Stadium that night, it should take you about 20 minutes from Dallas Market Center to rental car return. Allowing for time for returning the car and catching the bus to your terminal (15 to 20 minutes) and getting a boarding pass and thru security (both together should take 5 to 10 minutes), I'd say:

8 pm if you do not need to check bags
7:30 pm if you need to check bags (DFW operates with a 45 minute bag check limit which can be a royal p.i.t.a. when you factor leaving enough time to be in line "just in case").

Of course, if you get out a bit early, terminal D (the newest terminal) has nice (for an airport) restaurants so just get there, check in, and relax with a margarita.