The FOI Act states an obligation to reply promptly and within not more than 20 working days from a public authority receiving a request for information.
If the information sought falls within the terms of a qualified exemption and the public authority requires additional time in order to properly conduct a public interest evaluation the timescale for complying with the request is extended by a 'reasonable period'.
You must however tell the applicant within the 20 day period:
You should seek the advice of your departmental FOI specialist if you believe that a qualified exemption applies to the information request you have received and you anticipate that additional time will be required.