When it comes to "photoshopping" a model's likeness, where do you personally draw the line? How much is too much?

IMHO, if you can notice the picture has been edited, it’s too much!

it depends on many factors, like what it's of, what's the target audience.
the criteria is different for a beauty image for display at the makeup counter
and a baby picture on a desk.

also the extensive, but we'll done vs. minimal, poorly done, which was too much?

it depends on the purpose of the shoot.like for example a makeup editorial skin is generally goes through a massive retouching.but a boudoir candid can get away with minimal flaws correction. then there r things that u consider distraction.u gotta understand whats the purpose of the photograph.then ur retouching has to align with that idea.sometimes less is more;sometimes thats not the case.

How much is too much is not always an easy thing to determine.

However, this is where MM is useful, it’s a place to see plenty of other artists' images; look at enough photographs and you'll develop a personal sense of those alterations or changes that work and those that don’t.

I guess like most things in life, it comes down to personal taste. For me, there is something very special about the reality and natural beauty of some subjects.

There is no line.

Photography literally translates to "light drawing" or, "drawing with light".
A camera can be used. An image could be created from scratch on the computer. Both are "light drawing."
Any combination of the two is still "light drawing."

Imagination has no limits.

Humans often have "comfort zones" and attempt to define Art according to their own sense of safety.

There is no safety in Art.

Images have a purpose, purpose is a product of imagination. A creator of images can define themselves with their images but it does not provide definition of purpose or imagination of other Creatives.

I do see "Photographers" attempting to define other forms of drawing with light as "Graphic Design." They are incorrect but I have no time for absurd disagreements.