In about a month, I will be replacing a significant number of power
cords on a variety of metal and wood working machines.

So how long of a cord should a person replace them with?

It's a harder question to answer than at first glance. It's like asking
"How high should I build my workbench?"..it depends.

Of course one should replace the cord so the new one reaches the outlet
but how much extra cord should one allow?

Over the years, machines have come with a variety of lengths in
relation to their power cords so just replacing it with what it had may
not be a good approach. When copper was expensive, the cords got
shorter to save costs. And over the years, cords are replaced because
of damage or old age.

As a matter of good practice I will be installing wire adequate for the
currents needed, using grounded plugs, appropiate insulation types but
the simple question of "How Long?" is one I would like to have your
opinion on.

Thanks in advance for suggestions.

TMT