I had an injury that forced time off for 5 months. My FTP went from 315 w to 270 w. I started slow, with roughly 10% less volume and intensity workouts shorter than pre-surgery training. I gradually increased the volume, checking to be sure I didn’t overdue it and re-injure myself, for 5 months. By month six, I was doing pre-surgery volume of intensity workouts with still slightly shorter endurance workouts. Improvements started to come faster after that and one year later I was back to 300 w FTP. This year I plan to do full high volume endurance training and intensity work to get back to 315w FTP.