Go for 5 hours. First, Dfw-Lim is rarely on time; second, you get the morning rush hour (LA from Jfk/Miami/Madrid/BsAs, AM from Mexico, etc); third the immigration queue, baggage wait, and scanners upon exit; and last, check-in and security for domestic flight. On the other hand, take an evening flight..leave your bag in the airport and go explore for the day.