I'm doing the math, based on my own application at the beginning of October.
I'm starting in May 2007, as applicants from that period and prior have generally experienced normal processing times. In the month of May, there were already 743,000 cases pending. Presumably USCIS was able to cope with this load (most of the currently one million pending cases pre-existed the surge) and produce normal turnaround.
If you look at the actual number of receipts from the beginning of May to the end of September that exceed receipts from those months in the previous year, the excess workload (for my place in the queue) is approximately 157,000 cases. This excess should account for any additional processing time beyond what USCIS was able to maintain the previous year.
Post-surge (August through December) they've been processing (approving or denying) on average 69,000 N-400 cases per month. (This number could go up with planned staffing increases.)
At that rate, somebody who applied when I did should expect, on average, 68 additional days processing time beyond the previous norms. ((157/69)*30)
Of course, this doesn't take into account differences between DOs, being stuck in name check, etc. But it does make one question the estimates USCIS is giving.