Thanks @gak. Ok, so it’s a much newer endeavor than I thought. Yes, your push to github was a motivation for both LFortran and new Flang. 
I believe November 2017 for both.
I would say the clock starts ticking once you put the full effort into it. So the Classic Flang is 9 years old (perhaps a bit less if you don’t count the first year), and it’s quite mature, but didn’t quite deliver on the original hope to support all of Fortran. The new Flang is from 2017, but maybe you want to start counting a little bit later, so it’s around 6 years old.
And LFortran is from 2019 when I started working on it seriously, but I took a year off to help bootstrap fortran-lang, so maybe a little over 4 years old or so.
Now, the NAG compiler I believe was done in only about 1.5 years in 1990, maybe @themos can clarify, but back then it was only F77 and F90, which I think both Flang and LFortran support pretty well by now. It’s just that Fortran 2023 contains a lot more features.
If anyone knows time frames for other Fortran compilers, I would be interested. g95/gfortran, Intel, Cray, etc. I believe Intel said it took them 5 years to port ifort to LLVM (ifx)?
It all points to about 5 years to have something reasonable, and 10 years to full production.