Apparently it used to be 50 posts, not 5.
Only for like two days. That was before it was figured out how to implement the 4-hour time-online delay. 50 posts was necessary to create a proper barrier.
If you have ~4 hours of online time and your existing posts are good, then you
will be whitelisted on request. There's no need to post any more.
So after 4 hours and few posts, you have to ask to be whitelisted, its not something automatic? Its not very clear from the stickied newbie restriction thread. Would be good if this was clarified.