price differs based on airline.
for southwest, if some customer changes their $50 flight on that day to a different time / day their seat (at that price) opens up on the flight. best way to do it is to have a program like check for change constantly refreshing if that's your airline (about 45% of the McCarran Part 121 traffic).
for the major carriers like delta, united, american, us airways the 21 days probably holds true. we'd like to think southwest is in that "major carrier" category as well, as they are now double the traffic of us airways after acquiring airtran.
haven't really figured out spirit's price system yet but they're almost always cheap.