Not sure if any of these are 100% accurate, but this is what I've heard:
Generation 1: (RGBY) Development began around 1990.
Generation 2: (GSC) Started development immediately following Gen 1, which was released in 1996.
Generation 3: (RSE + FRLG) Production spanned roughly 2000–2002 and was released in Japan in 2002.
Generation 4: (DPP + HGSS) Was produced from about 2004 to 2006 and released in Japan in 2006.
Generation 5: (BW/BW2) Developed 2007–2010, released in Japan in 2010.
Generation 6: (XY + ORAS) Developed from 2011 to 2013.
Generation 7: (SM/USUM + LGPE) Developed from 2014–2016.
Generation 8: (SWSH + BDSP + PLA) Developed 2017–2019 and released 2019. (Not sure about PLA because it seems to have a lot more effort put in compared to the other games in gen 8)
Generation 9 (SV): Developed from 2019 to 2022 and released in 2022.
Not sure about development milestones though...
From this list, gens 3, 4, 6, and 7 have the shortest development times. Gen 2 (GS) had the longest development time, as it was released roughly 4 years after Gen 1.
If I'm wrong on any of this, please correct me!!!