Very good post everyone. First, it takes about 100deg of knee flexion before you can go completely around. This varies with the type of bike you use. Rocking back and forth is very common when starting this activity. You can go backwards easier because of the motion of the ankle. When going backwards, you tend to point your toes downward or forward in a recumbent. As far when you can start, it really depends on the angle of motion of the knee. A stationary bike usually takes about 105deg of flexion, again depending on the equipment.
I always use pain as my guide, if it hurts don't do it. The more aggressive the patient is, the slower the return to function. Everything in moderation.
I hope this helps,
David