Readability isn't just the inverse of code golf, but that's how it's often talked about. We want a clear picture in a few well chosen words, but writing padded with low effort filler is not clear, it's a drag. Whatever the answer is, it's not "shortest code possible" but it's also not "longest code possible". Given that, it seems quite plausible that the answer is "whatever you have learned to read is readable, and what you haven't, isn't".

We have ideas of a "grade 6 reading level", do we have such a scale for code? If not, why not?