There is not any limitations period for informal collection efforts, but there is a limitations period for bringing legal action to enforce an obligation claimed to be due. In NJ the limitations for the enforcement of obligations due under a contract is 6 years. If the collection agency were to file a lawsuit against you to seek to collect a contractual debt more than 6 years old, the NJ statute of limitations would provide you with a defense to that lawsuit. But you must file an answer to the court complaint (with the court clerk) and specifically raise statute of limitations as a defense. If you don't or you don't file an answer, the court might still enter a judgment against you because you waived the benefit of the defense by failing to formally raise it properly in writing at the proper time with the court. As another consideration, any third party debt collector that files a lawsuit against you to collect on a debt that is beyond the applicable statute of limitations may also be violating the Fair Debt Collection Practices Act and/or other federal and state consumer protection legislation. These violations would give you affirmative claims to raise against the collector in the lawsuit.
As with any legal question much depends on the particular facts. The answer set forth above is very general in nature and should not be relied on as being applicable or correct for your particular situation. Therefore, before making any decision about how to proceed, you should consult with an attorney who can provide you with specific advice tailored to the facts of your situation.