In the case of a subdivision for an area of 50 acres or more, the approving authority may grant rights referred to in Subsection B(5)(b)[1], [2] and [3] above for such period of time longer than three years as shall be determined by the approving authority to be reasonable taking into consideration the number of dwelling units and nonresidential floor area permissible under preliminary approval, economic conditions and the comprehensiveness of the development. The applicant may apply for thereafter, and the approving authority may thereafter grant, an extension of preliminary approval for such additional period of time as shall be determined by the approving authority to be reasonable, taking into consideration the number of dwelling units and nonresidential floor area permissible under preliminary approval, the potential number of dwelling units and nonresidential floor area of the section or sections awaiting final approval, economic conditions and the comprehensiveness of the development, provided that, if the design standards have been revised, such revised standards shall govern.