Say a 20' run verses 200' run, I would think the longer run requires a more exact length.
On the contrary, I'd say the
shorter the run, the more critical the match. It's all about the division of current, which is based on relative resistances; the lowest resistance carries the greatest share.
It's more important to maintain length (and thus, resistance) equality within a given percentage than a specific number of inches, so the longer the run, the less the relative resistance difference will matter.
Also, the closer to the limit of conductor ampacity your current ends up being, the more likely a small current difference can overload the conductor with the lowest resistance (including terminals, etc.).
Added: Plus, remember that the matching applies to the paralleled conductors of one given phase; phases need not match each other, although they likely will be close anyway.
I don't know a specific percentage to attain, but there are plenty of engineering types (but, we like them anyway

) on this forum who may be able to give you a figure.