The number of air miles needed for a free economy flight depends on the distance of the flight, the airline, and the fare class:
Distance: A short flight of 751–2,750 miles may require 10,000–12,500 miles for economy, while a medium flight of 2,751–5,000 miles may require 20,000 miles. A long flight of 5,001–7,500 miles may require 27,000 miles.
Airline: The number of miles required depends on the airline. For example, Virgin Atlantic requires 20,000 miles for an economy classic return flight during the standard season, but 40,000 miles during peak season.
Fare class: The number of miles required depends on the fare class.
Some airlines allow you to purchase miles directly, but this usually isn't the most cost-effective way. It can be a good alternative if you're only a few miles short of a reward. You can also earn airline miles by focusing on a couple of airlines that fly the routes you expect to fly. Points may expire if your account is inactive for a certain period, typically 12 to 18 months.