I started cycling when I was 40 having not even owned a bike since college. My n=1 experience is 1 year is a good time frame. For reference I’m 6’1" and 175 lbs (170lbs on race day). At my peak in 2019 (September) my FTP was at or above 300 at a weight of about 175 lbs for a 3.8 w/kg. I think the best w/kg I saw in 2019 was 3.85 when I weighted 168 lbs. Below are my first year FTP results starting with TrainerRoad which resulted in 100+ FTP gain:
Jul 9, 2019—295
Jun 22, 2019—290
Jun 16, 2019—287
Apr 9, 2019—293
Mar 14, 2019—287
Feb 12, 2019—282
Jan 15, 2019—276
Dec 18, 2018—265
Nov 20, 2018—250
Oct 23, 2018—260
Aug 6, 2018—235
Jul 11, 2018—193 (Started TR and 1st Ramp Test)