Definitely slowly at first! My break was in August '16. I was then doing PT as soon and often as they’d let me. Think I started putting weight on the leg (with a boot) in October. Started doing short (as little as 10 min. at first), easy trainer rides in between PT days in November and began to ever so slowly build up volume.
Biggest challenge at that time was I had almost no visible muscles remaining in that leg to start and my ankle flexibility (toe up/down) was reduced by the break (which was just above the ankle).
So come February of 2017 I was using the leg in everyday life, but it was still much weaker than before and than my good leg. One day that lack of muscle helped the meniscus in my knee pop when doing an otherwise innocuous squat in the kitchen. That led to a knee scope surgery and another bike hiatus until mid-March.
Once clear of that easier knee recovery I got back on the trainer guns blazing (so to speak!) and started to really increase volume. At first I didn’t want to do the intensity to do an FTP test so guessed a starting point of 165W (had been around 230W when I crashed). For first Sweet Spot Base blocks I simply increased my FTP 3-5% each week - which seemed to work really well.
In the first weeks after my injury I’d set a goal for myself of a really long gravel grinder in August the next year. So by August of 2017 I’d gotten back into some shape and my FTP was about 85% of where I’d peaked before. Completed the gravel grinder and was stoked! So that was about 6 months of training to get back to pretty good. (Included a screenshot of that time period training graph below)
Now the almost two years since have been a much slower progression from that point, until right about now I think I’m finally back at the FTP I’d peaked at before.
For me, that ankle flexibility thing hasn’t improved enough to even consider going back to CX , as running and jumping just still hurt too much and I’m too competitive with myself to try to do CX and not go hard in the off-the-bike moments.
Another thing I did during my initial downtime that was a big help was I taught myself and took up wheelbuilding. Kept my spirit in the game and was a good use of the emotional energy that needed to go into cycling somehow.
Happy to help with anything. Good luck!