We do know one thing for sure - that for AI it’s 0%
Once upon a time it was a contentious idea that humans had significant authorship in photographs, which merely mechanically captured the world. What % is the camera's independent inspiration?
Here, we have humans guiding what's often a quite involved process of synthesis of past human (and machine) creation.
The person using the AI doesn't matter in the equation. They aren't an artist, they're a monkey with a typewriter.
We're talking about the AI here, because it can generate the same images no matter which monkey with a typewriter is typing the prompts.
That's an opinion.
Does your opinion hold in all circumstances? If I spend 20 hours with an AI, iterating prompts, erasing portions of output and asking it to repaint and blend, and combining scenes-- did I do anything creative?
No matter how great the AI is, a monkey with an AI will never generate anything useful.
Is the result a copy, or perhaps a derivative work of the art in the training set?
Does the person using the model have authorship of the result?
Was it even okay to use the art to train the model and then share the resulting weights?
Are the resultant weights protected by copyright themselves?
I suspect the actual answers we'll come to on these topics will be full of nuance.