I mean the intervals between population hitting a new billion (6B to 7B, 7B to 8B) They intervals have been getting shorter and shorter due to exponential growth. Obviously there will be a last such interval when population peaks (no 9B to 10B if we never hit 10B), but I would expect them to lengthen due to slowing growth rates (ex. 8 to 9 takes longer than 7 to 8) instead of the last interval being the shortest one (zoom from 7 to 8 at record speed and then never hit 9) I haven’t dived into specific rates and forecasts tho.
The article does state 8B to the peak will be 2022 to ~2050, which is longer than 12 years, although I disagree with your reasoning that the time to peak must be longer than the time from 7B to 8B. It all seems very arbitrary.
You need to factor in the age of the population too. As large parts of your total population gets old you can have a sudden reversal.
It’s not a simple function of time and population. Fertility rate varies by country and more specifically by economic achievement. Population isn’t evenly distributed in either of those categories. We saw rapid gains due to China and then India modernizing. Now they are declining and peaked and its Africa’s turn but Africa is a continent made up of many countries in various states of development or regression which makes it very difficult to predict. If we take the happy path and all goes well with each country continuing to improve then they will peak quickly having the advantages of modern technology and three advanced economies investing in them.