Unless otherwise specified by the Board or by law, a variance shall expire if the applicant fails to obtain a zoning permit within one year from the date of authorization thereof by the Board or by the court if such variance has been granted after an appeal or fails to complete any erection, construction, reconstruction, alteration or change in use authorized by the variance approval within two years from the date of authorization thereof by the Board or by the court, if such variance has been granted after an appeal. The Board, upon written application and for reasonable cause shown, may extend the approval for an additional period of up to two years.