Cal. Gov. Code § 66451.17

If, within the 30-day period specified in Section 66451.14, the owner does not file a request for a hearing in accordance with Section 66451.16, the local agency may, at any time thereafter, make a determination that the affected parcels are to be merged or are not to be merged. A determination of merger shall be recorded as provided for in Section 66451.12 no later than 90 days following the mailing of notice required by Section 66451.13.