I had a manager once tell me during a casual conversation with complete sincerity that one day with advancements in compression algorithms we could get any file down to a single bit. I really didn’t know what to say to that level of absurdity. I just nodded.
It’s an interesting question, though. How far CAN you compress? At some point you’ve extracted every information contained and increased the density to a maximum about - but what is that density?
I think by the time we reach some future extreme of data density, it will be in a method of storage beyond our current understanding. It will be measured in coordinates or atoms or fractions of a dimension that we nullify.
I believe the general answer is, until the compressed file looks indistinguishable from randomness. At that point there is no more redundant information left to compress