I think it's 30 days AND 50 posts. At that time, you can also post links, and you gain access to the restricted area.
This is a protection thing. It means people can't come just to advertise their business by posting it in their signature. If someone sticks around that long, they become more a part of our "community", and we don't mind so much an "ad" in the siggy from one of "our own". The restricted area allows us more freedom to post information about our families that we don't want any lurkers reading. Also, we have a political restricted area. Presidential elections, especially, bring out the crazies! The last one, we had some lady who came just to tell us how we MUST vote. While many of us actually agreed with her discussion, we really resented the way she went about doing it, treating us as if we were totally ignorant.