Umm. Trick question? Maybe 30 days?
But the previous advise is good. Sounds like a legal matter, in which case you should consult with your lawyer. And don't wait until what you might calculate as Day 29. There are variables, such as "calendar days" or "weekdays." Does the 30 days begin running from date of mailing, date of receipt, or something else? Do your 30 days end upon your mailing (as confirmed by a postmark or receipt), or upon receipt by the other party.
Please see a lawyer as soon as possible. Not just regarding the calculation of the 30 days, but also to determine what your response should be.
Hope that helps.