PCT International Search and Preliminary Examination Guidelines
17.56 The written opinion of the International Searching Authority includes a notification to the applicant that in the event that he demands international preliminary examination he is invited to file with the International Preliminary Examining Authority his observations, to correct any formal deficiencies and otherwise to submit amendments to the description, claims and drawings before the expiration of the time limit within which he may file his demand (see paragraph 22.14).
17.57 Any written opinion established by the International Preliminary Examining Authority must fix the time limit within which the applicant must reply. The time limit must be reasonable under the circumstances. It is normally two months after the date of the written opinion. In no case may it be shorter than one month after the said date. It must be at least two months after the date when the international search report was transmitted along with the written opinion of the International Searching Authority. In no case should it be more than three months after the date of the written opinion.
17.58 Where a demand for Chapter II is filed, failure to reply to the invitation, whether from the written opinion of the International Searching Authority or from a written opinion actually established by the International Preliminary Examining Authority, may cause the international preliminary examination report to be established with a negative determination in relation to certain claims.