how far can you see with 20/20 vision

With 20/20 vision, you can see the details of an object at 20 feet that a person with "normal" vision (used as the benchmark) can see at 20 feet.

This means the following:

• 20/20 vision doesn’t have a concrete distance limit. It refers to the eye’s ability to see detail, compared to others with "normal" vision.
• You don’t need to stand 20 feet away to see it. You can see farther, so long as you don’t need to discern tiny details. You can see the sun, moon, a building on the horizon, a plane etc.
• There is no actual upper limit to the distance a person can see! The main limiting factor is other objects blocking your view, and, to a much much lesser extent, the light reflecting the objects.