....... and that's why one should be testing their own water. But by law I believe all municipalities must record their high/low chlorine and/or chloramine values. Most of that data can be found online. As an example as to how much of a swing can take place from one city to the next.
Some specific examples of actual chloramine levels found in drinking water located in the USA.
Portland Oregon 2015 Drinking Water Quality Report
Maximum levels of total chlorine residuals from chloramine detected: 2.02 - 2.72 ppm.
http://www.portlandoregon.gov/water/article/244813
San Francisco CA 2013 Water Quality Report
Chloramine residual levels average 2.2 ppm, with a high range of 2.9 ppm.
http://sfwater.org/index.aspx?page=634
San Diego CA 2013 Water Quality Report
Disinfectant Residual (Chloramine) distribution system average 2.2 ppm, with a high range of 4.1.
http://www.sandiego.gov/water/pdf/wq13.pdf
Massachusetts Water Resources Authority 2014 Report
An average of 1.8 ppm chloramine was reported (no maximum level given?)
http://www.mwra.state.ma.us/annual/waterreport/2014results/metro.htm
Houston TX 2013 Water Quality Report
An average of 1.96 ppm chloramine, and a maximum level of 3.8 ppm
http://www.publicworks.houstontx.gov/sites/default/files/images/utilities/wq2013.pdf
Kansas City 2014 Water Quality Report
An average of 2.27 ppm Chloramine, with a maximum level of 3.07 ppm
https://www.kcwaterservices.org/wp-content/uploads/2013/04/2014WQR1.pdf
Miami Florida
An average of chloramine residual in the main system of 2.5, with a high range of 4.2 ppm, and the Aventura (Norwood) system showing an average of 3.1 ppm, with a high range of 4.0 ppm
http://www.miamidade.gov/water/library/reports/water-quality-2014.pdf
Philadelphia
Philly has its tap water sourced from three different water treatment facilities, which across the entire distribution area average out at 1.98 ppm, with a high range of 3.8 ppm.
http://www.phila.gov/water/wu/Water Quality Reports/2014WaterQuality.pdf
Fairfax VA
An annual chloramine residual of 2.8 ppm, with a high of 4.1 ppm.
https://www.fcwa.org/waterqualityreport.pdf
Washington DC
Highest running annual average, 3.0 ppm chloramine, with a high range of 4.2 ppm.
https://www.dcwater.com/news/publications/DC_Water_Annual_WQReport_2014.pdf
The answer is there is no ONE SIZE FITS ALL for these types of questions, one has to have a local water disinfectant base number to start with before they can calculate the quantity of water conditioner such as Seachem Prime/Safe required.
I posted about this when Seachem changed their labels last year.
http://monsterfishkeepers.com/forums/threads/seachem-safe.651005/#post-7348490
To answer the OP's question, the good news is most water conditioners work instantaneously.