It is a mystery. I got my 2nd fp notice 6 months after the 1st one expired. my attorney told me that they stared scanning old cases fr expired 2nd fp a while ago. Whowevr, he also think that 2nd fp is initiated by an officer, since many of them are close to RFE. in this case you can expecr your case to be adjudicated within 1-2 months after fp/RFE. Even though he said that this is just his general observation, not a statement from USCIS, which he said becomes more and more mysterious, even to a lawyer.
About coming earlier dor 2nd FP, let me share my experiences. I received an 2nd FP notice scheduled for 12/23/2003. But I tried to come in earlier because I will be away for the holidays. At first I was turned away rudely. Luckily, the manager/supervisor of the ASC was close by and interfered, saying that I am welcome to come at later date if I cannot make it on 12/23. She also saidd taht it probably better to come later, since with the delay they have been requiring more and more people to get 2nd FP. WHen I told her that I was coming for my 2nd FP not the 1st one, she asked me why I did not tell her earlier. (???) and then she took my FP and wished me luck. Thankfully that the supervisor was much more courteous/knowledgeable than the stupid rude officer who turned me away the first time.
Note: My case RD was 9/01