It is 5 days/5 posts as Jim K states. This lets us weed out the spammers that get through our first few lines of defenses (just got one today). But it is 50 to be able to post classifieds.
Tradition is to make it to those 5 posts by wowing us with some hot aviation-themed T-shirt or a ludicrously underpriced Bose A20. Bonus points if your avatar pegs the thirst-o-meter.