Question

What is the proximity required for a magnet to affect a metal object?

Answer 1

You need to define "effect". It also depends on the strength of the magnet, but if you're talking about something like a fridge magnet or something like that it's a very small fraction of an inch before the effect is negligible.

Answer 2

That is a much larger question than I suspect you think. The answer is different for every magnet and has to do with something call flux. Think of it this way. The actraction to a magnet increases exponentially as the metal gets closer. So your answer is; as the metal object moves away from the magnet, the strength of the force heads towards zero. Hope that helps in some way.