I found if your case hasn't been assigned to an officer before FP expiration, then you get it exactly 15 months.
If your case has been assigned to an officer, then the date varies, but always after 1 week after FP expired. Let's say you did FP on March 8.2002, then you probably will get second FP after June 15, my optimistic estimate is later June.
Note, INS use received date as FP expiration date.