Grounds for challenges to a will

In NJ typically there is only 4 months to file a Superior Court complaint to object to a Will which is already filed with the Surrogate
See Rule 4:85-1. Complaint; Time for Filing
If a Will has been probated by the Surrogate's Court or letters testamentary or of administration, guardianship or trusteeship have been issued, any person aggrieved by that action may, upon the filing of a complaint setting forth the basis for the relief sought, obtain an order requiring the personal representative, guardian or trustee to show cause why the probate should not be set aside or modified or the grant of letters of appointment vacated, provided, however, the complaint is filed within four months after probate or of the grant of letters of appointment, as the case may be, or if the aggrieved person resided outside this State at the time of the grant of probate or grant of letters, within six months thereafter. If relief, however, is sought based upon R. 4:50-1(d), (e) or (f) or R. 4:50-3 (fraud upon the court) the complaint shall be filed within a reasonable time under the circumstances. The complaint and order to show cause shall be served as provided by R. 4:67-3. Other persons in interest may, on their own motion, apply to intervene in the action.

4:85-2. Enlargement of Time
The time periods prescribed by R. 4:85-1 may be extended for a period not exceeding 30 days by order of the court upon a showing of good cause and the absence of prejudice.
Commons grounds object to Will is undue influence.      

Undue Influence to challenge a Will or Power of Attorney      
  A grievance based upon undue influence may be sustained by showing that the beneficiary had a confidential relationship with the party who established the account. Accordingly, if the challenger can prove by a preponderance of the evidence that the survivor had a confidential relationship with the donor who established the account, there is a presumption of undue influence, which the surviving donee must rebut by clear and convincing evidence.