My therapist reminded me of the 10% rule. When you can do something comfortably for a few days, you can increase your time/distance/reps (whatever is appropriate) by 10%. Like Jaycey suggested, start with the bike at no resistance for 10-15 minutes. When you have no ill-effects the next day, you can increase by 10%. Keep doing that gradually. As your knee heals, you will be able to add more exercises (using the same approach).
I was quite frustrated with my left knee's progress in January. I was still having quite a bit of pain walking up stairs. I've noticed now, at 23 months out from surgery, that knee is FINALLY doing stairs without major discomfort. Sometimes our progress is frustratingly slow, but it is happening. Hang in there.