Only your doctor can answer that.
30 minutes
Talking about riding speeds and riding distances on a stationary bike is rather pointless because the bike isn't going anywhere, at any speed. If you're riding for exercise, count the time instead.
Since a stationary bike doesn't move, there's no way it can travel 20 miles. Now, if you're asking how long it would take to ride twenty miles on a bicycle, it depends on speed, wind, and other factors. Typically a bike maximum speed is about 20 miles per hour. So it would take about an hour to ride 20 miles on a REGULAR moving bicycle.
Entirely pointless question as there aren't any steps in bike riding, and a stationary bike doesn't go anywhere. But making some rough comparisons to real life riding a 20 mile ride under average conditions can be anywhere between 45 minutes and one hour 20 minutes, depending on the rider, bike and course. A well accepted recommendation states that the cranks should turn at a rate of 80-100 turns/minute, which gives you around 6000 turns for the ride.
No, it is illegal to ride a dirt bike on the street but you can ride it on a trail or dirt bike track.
That's all down to the skills of the rider. There's even the art of trackstanding, where you stay on the bike while remaining pretty much stationary.
Get a bike and ride it
You ride a bike because it has to be balanced by the operator.
It depends on how fast you can ride a bike!
This practice is to mainly avoid muscle cramping that can occur when strenuous activity is immediately followed by rest. Most professionals refer to this as "cooling down".
This is the kind of question for which you really should consult your physical therapist, rather than a web-site. If you have healed well from the surgery and are in good shape, then yes, you can ride a mountain bike with a knee replacement. But I do not know what kind of condition you are in.
i keep the bike on tand and ride on it , then handle is no bar