Read this: https://bitcointalk.org/index.php?topic=178608.0. You need 30 activity points to rank up, and basically you can acquire no more than 14 activity points in 2 weeks. However, since you registered in December, at present you are limited by the number of posts, not time. Ranks above Jr. Member also require merit: https://bitcointalk.org/index.php?topic=2818350.0.