In the case of a subdivision of or site plan for an area of 50 acres or more, the board may grant the rights referred to in paragraphs a, b, and c above for such period of time, longer than three years, as shall be determined by the Planning Board to be reasonable, taking into consideration (1) the number of dwelling units and no residential floor area permissible under preliminary approval, (2) economic conditions, and (3) the comprehensiveness of the development. The applicant may apply for thereafter and the Planning Board may thereafter grant an extension to preliminary approval for such additional period of time as shall be determined by the Planning Board to be reasonable, taking into consideration (1) the number of dwelling units and nonresidential floor area permissible under preliminary approval, and (2) the potential number of dwelling units and nonresidential floor area of the section or sections awaiting final approval, (3) economic conditions, and (4) the comprehensiveness of the development; provided that if the design and improvement standards have been revised, such revised standards may govern.