2d year riding with a PT. FWIW, I have seen about a 20 percent jump in FTP in the course of three months. Although I did cycle last season, injury put a stop to it and I pretty much started over from close to scratch. This is not on massive volume, really: 2-3 hour to hour and a half training sessions during the week targeted at FTP, and then one longer ride on the weekend. I have finally come close to the "magic" 3w per kilo (magic for me, anyway), and am putting out normalized wattages for half an hour or more that I could not sustain for 20 minutes or less--in groups with very high motiviation.
The progress is undeniable-it's just that the effort has to be consistent, and the PT helps dial in the effort and measure fatigue. I strongly recommend reading the "it's killing me" thread. Tyson began at 20 min at 150w, and rose to 300. At seventy years old.
Now that the weather has shifted, I'm hoping for four days a week. (The weather this weekend is not, sadly, cooperative.).