When the spec for software you're developing changes after you've finished developing it, what percentage of the time do you need to significantly refactor your

When the spec for software you're developing changes after you've finished developing it, what percentage of the time do you need to significantly refactor your logic instead of just making a minor tweak/extension? Might not be a perfect measure, but it's a very noticeable one that in my experience has been going down sharply as I've gained experience.