I was in Mexico City two weeks ago. Traffic was light when I arrived; it was a 25 minute taxi ride. On my return, it was closer to a 45 minute ride.

If you go, be sure to go to the pyramids. It's easy to rent a car and drive to them. They are on the same side of the city as the airport.

If you rent a car for multiple days (and I DON'T recommend driving in the city), be aware that cars are banned from driving in the city one day a week based on the last digit of the license plate. The airport is considered in the city, so the rental car must have a license valid both for the day you pick it up and the day you return it.