37 CFR § 42.120 - Patent owner response.
(a)Scope. A patent owner may file a response to the petition addressing any ground for unpatentability not already denied. 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.