Does there exist some more reliable estimates? Does the USCIS give some expected waiting times, as they do for some other processes?
https://egov.uscis.gov/processing-times/ gives some estimate for the N400 processing time for each USCIS field office, but no breakdowns for each step of the N400:
I am guessing that the end of the N400 processing is the oath ceremony day but I'm not sure.
https://egov.uscis.gov/processing-times/more-info explains how to read the two numbers:
Estimated time range: This column lists case processing times as a range for select forms and locations. The lower value of the range is the time it took us to adjudicate 50% of our cases. The upper value of the range is the time it took us to complete 93% of our cases. See below for a description of how these ranges are calculated.
Update (2022-09-03): since around May 2022, the displayed number reflects the time it took the USCIS to complete 80% of cases over the past 6 months.
We generally process cases in the order we receive them. This processing time is based on how long it took us to complete 80% of adjudicated cases over the past six months. Each case is unique, and some cases may take longer than others. Processing times should be used as a reference point, not an absolute measure of how long your case will take to be completed.
Historical data can be found on https://egov.uscis.gov/processing-times/historic-pt but isn't broken down per USCIS field office.
Announced on March 29, 2022: https://www.uscis.gov/newsroom/news-releases/uscis-announces-new-actions-to-reduce-backlogs-expand-premium-processing-and-provide-relief-to-work the USCIS aims at reducing the N400 processing time to 6 months by the end of FY 2023.
Some websites or phone applications provide their own time estimates. E.g.:
Some websites track applicant cases, e.g.: