If you meet the requirement under any of the categories (family based, employment based, asylum etc.) you can apply anytime. Otherwise, I haven't seen any law that allows automatic adjustment of status to permanent resident because someone stayed here for 5 yrs or 10 yrs or 20 years. Once you get PR status, you can apply for citizenship after five years. Again, there are certain requirements including a civic test that you must pass to be a citizen of US.