It's really hard to tell as it depends on a lot of circumstances. On roads, and at a moderate effort I can do it in about 45 minutes. A racer, going flat out could probably make it in 25-30. Most would be able to do it in about one hour.
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Depends on a lot of things. A fit rider can do 20 MPH for several hours under reasonably easy riding conditions, so maybe as little as one hour. But someone who's less fit, has a poorer bike, or rides somewhere more difficult - anything between 2-3 hours.
Depends on the human, the bike, the terrain and the weather. I could probably do it on my commuter bike in 40 minutes of riding, assuming good weather and a flattish course. A group of more dedicated riders on betters bikes should be able to polish it off in less than 30 minutes. Someone else may need more than one hour.
If that is 20 kilometers it can be around 30 minutes, but would depend a lot on the course.
There's no telling. For road bikes in flattish terrain it'd be less than 30 minutes. For MTBs in flattish terrain it could be 45 minutes. For MTBs in difficult terrain it could be one hour.
There's no set speed for bike riding so it varies. A good racer on fairly flat roads should do it in 30 minutes. I'd do it in about 40. Some may need a whole hour or even more.