37 C.F.R. § 42.120 : Patents, Trademarks, and Copyrights — UNITED STATES PATENT AND TRADEMARK OFFICE, DEPARTMENT OF COMMERCE — GENERAL PATENTS — TRIAL PRACTICE BEFORE THE PATENT TRIAL AND APPEAL BOARD — INTER PARTES REVIEW —AFTER INSTITUTION OF INTER PARTES REVIEW — Patent owner response

(a) Scope. A patent owner may file a single response to the petition and/or decision on institution. A patent owner response is filed as an opposition and is subject to the page limits provided in § 42.24.
(b) Due date for response. If no time for filing a patent owner response to a petition is provided in a Board order, the default date for filing a patent owner response is three months from the date the inter partes review was instituted.

37 C.F.R. §42.120

85 FR 79129, 1/8/2021