How long does it take the narrator and his neighbor to mend the wall

it takes the narrator and his neighbor a day to mend the wall between their properties. We know this, because the narrator states:

And on a day we meet to walk the line

And set the wall between us once again.

Explanation:

Someone who moves his or her body with a perfect balance and that uses correct movements that allow them not to miss a step or beat is someone who has a great coordination, and that can make this kind of movements in a quick way. This is a definition of someone who is agile, which is the correct answer to complete the sentence.