This is one of those things that there is no hard and fast answer it depends on what the image is, and what it will be used for ie. web gallery or large print. As an example for a female portrait, retouching for a web gallery will probably be no more than 10 to 15 min per image and I will use a plugin for skin smoothing. After final picks are made the images that will be printed will get much more precise retouch in the neighborhood of 1 to 3 hours per image with no skin plugins used ( just don't like how they hold up in large prints ). I have a few friends who are professional retouchers and for high-end clients they can spend upwards of 15 - 20 hr for an image.
