Short answer. Twenty calendar days from notice of the order admitting the supplemental complaint, unless the court sets a different period, per Section 7 of Rule 11. If you don't file a new or supplemental answer, your existing answer to the original complaint automatically serves as your answer to the supplemental complaint too.
What the law says
A supplemental complaint may be answered within twenty calendar days from notice of the order admitting the same, unless a different period is fixed by the court. The answer to the complaint shall serve as the answer to the supplemental complaint if no new or supplemental answer is filed.
Rule 11, Section 7 — Answer to supplemental complaint. Read the full provision →
The default twenty-day period
Section 7 sets the general answer period for a supplemental complaint at twenty calendar days, running from notice of the order admitting the same. This ties the deadline not to the filing of the supplemental complaint itself, but to the court's order admitting it — meaning the clock only starts once the court has actually accepted the supplemental pleading into the case, and the responding party has notice of that order.
The court can set a different deadline
The twenty-day figure is a default, not an absolute: Section 7 allows a different period to be fixed by the court. This gives the court flexibility to adjust the answer window based on the circumstances of the particular case — for instance, if the new matter in the supplemental complaint is especially complex or requires more time to investigate and respond to.
Silence has a built-in fallback
Perhaps most practically, Section 7 provides that if the responding party does nothing — files no new or supplemental answer at all — their answer to the complaint shall serve as the answer to the supplemental complaint. This is a safety net that prevents the supplemental matter from being treated as unanswered by default; the existing answer is deemed to cover it, though a party who actually wants to specifically respond to the new matter should still file something new within the deadline.
Why filing a new answer is still often the safer move
The fallback answer exists to prevent default, not to guarantee a full defense. The original answer was written to respond to the original complaint's allegations, and it may say nothing about facts or claims that only appear in the supplemental complaint — a new theory of liability, additional damages, or events that occurred after the original pleading was filed. Relying on the old answer means those new matters are effectively left unaddressed, which can weaken the responding party's position later. A new or supplemental answer lets the party specifically admit, deny, or raise defenses to what the supplemental complaint actually alleges, rather than leaving the court to infer a position from a pleading drafted before the supplemental matter existed.