I'm 5 months out of a total knee, and my advice to you is be really careful getting back on the bike this early. Mountain biking was my main recreational activity before my knee surgery, and I was so eager to get back to it that I over did it in PT, which resulted in significant hamstring and hip issues. I"m now dealing more with those issues than my knee, so be careful not to cause more problems than you start with.
I was able to get full rotations on the bike within a month or two, but I've found that it causes my hip to shift out of alignment and cause awful deep hip and/or back pain. I'm not sure if it's sciatica or not, but it's a real problem. I think I don't know how to just spin with no resistance on a bike, so I find myself trying to ride too fast and to turn up the resistance so that it starts to feel more like a workout. But it's not doing me any good at all, so this last month I've basically stopped trying to get back on the bike. I know it will come eventually.
This surgery is a huge mental challenge for those of us who enjoyed "working out" prior. I've had to really adjust my mindset, and seek happiness in the quiet around me. Patience is not my strength, but this surgery is forcing me to incorporate it.
Good luck!