I would give yourself at least 1/2 hour. From leaving the park, to get to your car, and on the highway, I'd say about 45 minutes total. Much of it also depends on what time of year, and what is going on. During
HHN, Mardi Gras, summer, and holiday crowds, those getting in and out of those garages can be more ticks off your time than you think. So, YTMV.
One year, we were coming from Sea World area to eat dinner on Christmas Eve in CityWalk. There was a huge sea of people in CityWalk, as the parks were closing in 15 minutes before we had arrived. Speed was not our friend.
One time, we were going to Disney, but stopped at CityWalk one morning to meet up with friends in CityWalk. There was a mass of people still coming into the park around 10 that morning.
Even at an 8:30 leave on a 9pm closing, there are more people in CityWalk that want to enjoy the places in there. Many seek food and/or shopping as it gets closer to park closing time.
Guess that is why no one can absolutely say. And in the second week of January, one evening, we went from Downtown Disney to CityWalk, and it took us almost an hour, as I4 can be slow any time of the day or night. Construction and cars that don't know lanes and where they want to get off.
Good luck, and hope the meal you want to go to is a great one.