Usually re-touching is an add-on. Say something like this:
basic retouching, included in a standard shoot, includes face retouching and global corrections, such as skin-tone or overall smoothing (or whatever you are fine with including).
Additional retouching is $100/hr, rounded up to nearest half hour. Ask for an estimate.
The first question for yourself is what do you WANT to be doing as a photographer. The second is, how long would it take for a "difficult" photoshoot to accomplish that editing? Then budget accordingly, and specify to the client that things beyond those criteria are extra.
My shoots would fall in one of two categories:
No editing - Client pays for my shooting time and gets all digital files unedited, or VERY lightly edited (delete blurry, crop a few key pictures).
Budget hourly for X times the length of the photo shoot, where X is some function of how much editing is appropriate. A wedding, I'd say 3x the photo shoot length. A portrait shoot can be 2x. So I'd charge a wedding with 4 hours of photography, the price of 12 hours of labor. The 8 is pre and post production time, and I try to honor my time this way. A portrait shoot with little advance planning, and especially at my house/studio, might take only 45 minutes, and I'm charging 1.5h, making sure I only edit the entire shoot in 45 minutes. These are very global edits. Delete most pictures that are mediocre or worse. Batch color and exposure correction, global smoothing. Inspect each picture for a need for any major custom adjustments (e.g., one over-exposed picture), and fine tune 2-3 of the best pictures for showcasing.
In practice, I find its hard to stick to my time limits for editing, and I don't like charging too much, so most of the time I try to sell my clients on a 0-edit version. It gives me lots of peace-of-mind. These days of social media, most people are at least a little familiar with making their pictures look better, and I then don't need to carry the burden of 'how much is too much skin correction?'