Say if i reach 120+ posts till the end of this month then will i be ranked "Full member" in the end of the month itself or i have to wait till november?
As --Encrypted-- explained, you'll have to wait until November, 24, before you can become a Full Member (and that, assuming you post at least once in every two week period, and make at least 120 posts in total). For reference, the next two week period starts this Tuesday, the 15th of September.
To clarify, and repeating what others already pointed out, your forum rank doesn't depend on how long ago you registered, or even on your post count alone; it depends on your activity, which normally increases at a maximum of 14 points every 2 weeks, as you make posts. In your case, as you have not posted enough to gain all the activity you're entitled to, you'll be able to get a bit more (up to 42 activity points this period).
