What is the test for establishing that an exclusive use agreement is enforceable for a period of enjoyment?

"The protax trend has found courts testing the requirement of a reasonably certain period of enjoyment by an examination of the agreement as stated in writing and [italics in orig.] the history of the relationship of the parties, thereby finding durability because of the passage of time even though the agreement may have been cancelable at the will of the parties. The requirement that the use must be exclusive means that it must not be [185 Cal.App.3d 383] one shared by the general public and, at least until cancelled, must be enforceable against the public entity which permits the use. As previously indicated, the valuable private benefit may be income, or merely a place to live incident to employment." (Freeman v. County of Fresno, supra, 126 Cal.App.3d 459, 463-464, 178 Cal.Rptr. 764, italics added.)