A pragmatic answer is when it is good enough for the end product you are looking for. Ultimate use dictates how much time and effort, as a hobbyist, I am willing to spend on getting it to be “good enough”. Something that I will be printing on large format paper, will get a lot more effort than something I am going to post on the web.
My criterion for “good enough” when I view something at is final size and don’t see any tangible improvements, when I continue to work on something.
Because I know that I am going to PP many of my shots, I actually shoot with that thought in mind. What that allows me to do is concentrate on composition, knowing that I will be able to fix things in post. As an example, I remember taking a shot of a building that had telephone lines running in front of it and a stop sign obscuring part of it. Was there and we struck up a conversation; he was struggling to “get it right in the camera”. Yes, he did manage to show me a shot, where he had more or less eliminated the things that both of us did not like in the scene. Bottom line; he had to compromise composition to reduce / eliminate the need to PP his image. Frankly, in my opinion, he did not have a very good image.
When I say I shoot with PP in mind; that is exactly what I did in the images I showed in the example I showed further up this thread. Here is another example; this shot of Centre Block of the Canadian Parliament Buildings, is another example of what I mean by shooting with PP in mind. This is a composite that is made up of around 7 different images; with lots of people in the foreground. The sky was dull and overcast; so I needed to bring out the clouds, and I used a grad ND filter while shooting. If I were to shoot this again, I would have taken a few more shots without the grad, to make the PP a bit easier. As you can see, the crowds are missing and I managed to remove the effects of the grad on the Peace Tower. The face of the clock was a separate shot to make it show up better. The flag was taken from a shot I had taken some months ago, when the wind direction was better and it was blowing harder. This is not a HDRI image, buy the way.
When I refer knowing that it is "good enough". There are still people in the picture that I did not take out, because they really are not noticeable in the final product.