rubbing through the paint

How hard is it to rub through the paint with liquid super hard shell turtle wax? I used it to rub out some light scratches and scuff marks.

Depends on the thickness of the paint, the applied pressure, etc. There are many variables to consider. Some paint might where through quickly, others might take awhile.

How can I tell if I went too far?

When the area becomes really dull, or you see the primer. By then its too late. Talk to Mark see if has any tips on this deal.

Usually when I buff, I buff the paint jobs I do. I know how much paint I applied, and how easily I will rub through to the primer.

Well, it is very shiny and smooth. Looks the same as the rest of the car. It is a new 04 Civic.

Im not a huge fan of the turtle waxes , but if it looks good thats all that matters. Just remember a big rule in polishing , if you polish a little scratch on a door for example you gotta polish the whole door if you dont there could be reprocussions.

Why is this?

well your renewing the portion of the door with the scratch after it is hidden or polished over the rest of the door will look duller than the spot you just worked on.