It's not an airline based number it's determined by which country you are visiting - some are 3 months, some are 6 -
http://traveltips.usatoday.com/count...ty-100788.html
This happened to me on my way to Amsterdam on Lufthansa and I was not allowed to board - they are very strict on this - they stated if they let me get on I would just be turned away at customs when I arrived in the country as they check this - it's not an airline thing - also United did not re-book me without me paying change fee + difference in fare - and I was Gold status at the time
This is a better site to check
http://travel.state.gov/content/pass...h/country.html