This is a really good question!
I believe the general answer is, until the compressed file is indistinguishable from randomness. At that point there is no more redundant information left to compress. Like you said, the ‘information content’ of a message can be measured.
(Note that there are ways to get a file to look like randomness that don’t compress it)
For me that’s under Internet > Saved Networks > network name > Share, instead of doing it from the main list of networks