How long can I marinate pork with a dry rub?

I had a feeling "it depends" might figure into answer...I do appreciate the extra info, though -- it helps the guesswork of seasoning be more like educated guesswork.

Honestly, you'll only know after tasting if the meat is too salty or not salty enough. It takes some experience with the rub you're using to know how much you should use for any given cut of meat. A roast can take *a lot* more than a rack of ribs.

"It depends" is probably not the kind of answer you were looking for. Maybe this will help a little: I might use 2 Tbs. of kosher salt for a 4-pound beef roast before letting it stand for 24 hours. Ribs receive a liberal coating of rub but it's probably only 1/8 kosher salt. In other words, not much more than if you were seasoning just prior to cooking.

ChefOno, any tips on how you know if you've used the correct proportion of salt to meat?

You should be fine. Assuming you used the correct proportion of salt to meat, the additional standing time won't make any difference since equilibrium will have been reached long before.

(By the way, the term marinate denotes a soaking in a liquid preparation.)