How far away should I be from my subject in order to avoid perspective distortion?

Could you explain how it is done?

I have a DX Nikon camera. If I shoot with a 25mm, 50mm, and 85mm lens then how far should I be from the human in order to avoid this factor?

It's not going to happen.

Ideally, you should keep your camera absolutely level, but the temptation to lean back to include the topof the subject is generally too great. That's what shift lenses are for.

For portraits? It's not fixed for every given focal length. You take tons of shots at different distances at various aperture sizes using each lens to find that out yourself what the optimum distance is.

For architecture? You don't think about it. You buy a perspective control tilt-shift lens.

As a generality the "normal" perspective is 50mm for full frame or 35mm for DX. But really any focal length can have a kind of perspective distortion. What kind and how much depend on focal length and distance. What is "good" or "bad" depends on what you are doing and want. Wider angles make foregrounds loom larger and backgrounds shrink.

Stepping back and using longer focal length makes background loom larger and creates more compressed perspective.