I would like to know the law in New Jersey for late fees. How many days grace is the law in New Jersey for paying rent before a late fee can be charged?

Generally there are no laws requiring a grace period. A payment is late if not paid when due. Any grace period would be written into your lease.

Up to your contract, from one day to 10 normally, 5 is the most common number of days used. But many have no grace period what so ever, expecting rent to be paid prior to the date due.