Which is correct use of the apostophe in minutes?

Minutes walk to the building. or Minutes' walk to the building. or Minute's walk to the building.

As in it is several minutes walk to the building...

You can use the apostrophe immediately after the s of minutes but need not add an extra s. That would be incorrect.

"Ten minutes' walk" is fine. "Ten minutes's walk" is incorrect.

Because it is a plural, adding another "s" is not correct.

So the answer is:

several minutes' walk to the building...

Possessives and apostrophes has some good rules and explanations for this.

Standard orthography uses minutes' here. To see why, note that in the singular, we say "one minute's walk", not *"one minute walk". So we need the possessive form, which in the plural is minutes'.