Summary: From an artistic perspective, there is nothing like "too much Photoshop." If you or your client want a highly retouched result, then there is nothing wrong with that. From a time management perspective, it looks very different. The first three minutes of Photoshop may improve the photo quite a lot. The following 3 hours of Photoshop don't add all that much to your picture. So if you spend more than 3 minutes per photo in post-production, then it's too much, unless you are a retoucher.
