You need 6,000 to 20,000 miles for a free one-way flight, depending on how far you're flying, which airline you're using and the dates of your travel. A one-way international flight will usually cost 35,000-50,000+ miles. Airline miles and points are generally worth around 1 cent each, but their value can vary.
Miles and points earned on co-branded hotel and airline travel rewards cards do expire usually if you are not active in redeeming them for 12 to 24 months, depending on the terms of the hotel or airline loyalty program.