I have a friend who did learn to ride a bike in an hour. I was extremely surprised, but he is the fastest learner I know so I don't think it's typical.

Frankly it is easy as an adult, even if falling is probably worse.

You would not believe how uncoordinated children are, and how much better you are at using your body a few years after it finishes growing and you've had some time to get used to it.

(I mean, developing proper instincts about stopping and turning can take some time, but just mastering going slow and not falling down isn't too hard.)