Federal NICS checks are instantly approved as soon as the dealer hits the submit button, only if the name submitted is not flagged. If not instant, the check goes into a RESEARCHING status where the identifiers are compared to the data bases searched. The time for this has recently been taking about 30 minutes. Then result could be APPROVED, DENIED, or DELAYED. The DELAYED has been taking 1 to 2 days before getting an approval. After 3 business days (Brady Law), the DELAYED goes into an OPEN status, where the dealer can transfer the firearm at his discretion. After 30 days, the status goes away. Denials are self explanatory.
Of course, in NJ the clock does not start until an NTN is issued to the dealer for a NICS check.