Well, in essense, everything is down to the original image. You can only work on what is already there. Don't get me wrong, the pictures look vastly different from what they do in the camera, but the interplay between light and shadow is what allows you to work on a picture so heavily.
But to answer the question of how much I play about with them, I would say a lot. But, the changes are quite subtle, and each change builds upon an element of the picture that already exists.
Essentially, I just change **** in photoshop until it looks good. Sometimes it takes ten minutes, sometimes it takes an hour. If it's paid work, it might take me three days.