I had a total kneee replacement in May of last year. It got infected, had to run antibiotics. Develoed a bunch of scar tissue that was blocking ROM, so they went back in August with the arthroscope and got that out. Recovery has been slow but steady since.
I thought I would maintain my upper body no problem, but having your leg cut off and reattached isn't that minor of a procedure. I just really started training upper body effectively in January. Coming along OK with that.
As for the leg itself, some days it feels a lot stronger than others. I can do a variety of squats, with very little weight. I do hacks with one plate per side, free squats with 135, smith machine with a plate plus a 25 per side, and DB squats with 50's. I use a low bench for a depth guage,(except hacks) and I can get deep enough, barely below parallel. I shoot for a minimum of 12 reps on all sets.
I do leg press for higher reps, 20+, using partials in the bottom part of the move to warm up.
The trick on squats and leg press is to keep focused on the replaced knee/leg to do its share of the work. Takes some focus once the set starts getting difficult.
Hardest thing for the replacement knee is extensions. Some days I feel I'm getting stronger with them, some not.
I've had major surgery on both shoulders and one elbow before the knee. Took at least a year after each to get back training like I want. This isn't going to be any different.