The number of miles required to purchase a free airline ticket will vary depending on the frequent flyer program, where you are traveling to/from, the timing of your flights, seat availability, and the class of service.
You should generally expect to pay no less than 25,000 miles for a round-trip domestic ticket within the continental U.S. unless there is a fare sale or saver-level award available. You may also have to pay much more depending on the factors previously mentioned.