The airlines determine mileage from GPS as well as geographical data. As for how many miles you need before you fly; that will depend on the airline that you fly with and their frequent flyer rules. Each major airline has some type of frequent flyer program with different ways to earn miles. Usually for every mile that you fly, you get one mile. There are different levels of frequent flyers, so the more that you fly, the more benefits you will get to include bonus miles. Once you start to accumulate miles, you can use those miles to fly for free. The region that you will be flying to, determines how many miles you will need to use to get the tickets as do other restrictions. The actual mileage will depend on your exact starting and ending locations. It should take a little over 20 hours, depending on weather and traffic conditions. Flying between the two locationss airports can be longer or shorter, depending on airport location and actual route chosen.