How long does it take for mail to get to Iowa from New Jersey?

There is no way to know the answer to your question, in the case of ordinary first class mail. There are cases where mail from post offices 20 miles apart takes takes weeks to get deliv delivered. All mail is now sent to regional sorting station and then sent to other regional stations where it is sorted for local delivery.

However, the postal system, overall ,does pretty well- most letters from Iowa to NJ probably arrive within 2 0r 3 days and more than 4 would be an oddity.