In evaluating an application for a conditional use, the Board of Supervisors shall apply the guidelines and procedures set forth in the Pennsylvania Municipalities Planning Code. An approved conditional use shall commence within 18 months following the date of approval, unless the Board of Supervisors has established a different completion date at the time of approval. Also, the Board of Supervisors may grant an extension of time for any completion date if the landowner or his agent requests such an extension and if good cause for the extension is shown. There are no other exceptions to this rule. If, at the end of the eighteen-month period or extended completion period, the conditional use is not completed, and if no extension has been granted, the approval of the conditional use shall be null and void. Upon approval of any conditional use by the Board of Supervisors, any prior approved conditional use for the same tract of land shall become null and void.