Generally speaking, it takes about 2-3 weeks to set-up a DBA in New Jersey. Some counties offer an expedite service that will get your DBA filed at your County Clerk's office in within a few days. However, the paperwork being processed at your County Clerk's office is only a part of the process. In most cases, a notarization is required on the initial DBA application, and the county offices do not always send your paperwork back the day they process it.
Please also keep in mind that in many cases DBA's are limited to operating in their county of designation, and offer absolutely no asset protection. We always suggest consulting with your attorney/accountant to decide if a DBA matches your business aspirations and functionality with practicality.
I hope you found this answer to be of some assistance.