I've actually had a held reservation on AA cancelled within 2 hours of creating it. When I went back to purchase, I was told that the price I had reserved was no longer valid and the $450 fare was now $600. I was able to speak with a supervisor and was given the opportunity to purchase the original fare by phone if I purchased immediately.

That experience did caution me to the fact that "up to 24 hours" essentially means "for as little or much time as AA wants." Although I was able to purchase the fare after talking with a few folks on the phone, it's probably possible to be denied the purchase. After all, 10 minutes falls under the umbrella of "up to 24 hours."