The truth is that while they should process your case right away there is no guarantee that TSC will do it. The answer is typically that your receipt date is later than the receipt dates that are being processed. Last time i checked they were processing June/July 09 receipt dates. I applied for 485 in Oct, it was pre-adjudicated in in Dec but not approved at that point even though my priority date was current and all documentation, name check and finger print were in order. So I'd say if you are lucky your case will get picked up in a month or two, otherwise I'd say it will atleast take 4-6 months on an average.
hope that helps