George knows that he and Lennie must stay at the ranch in order to get a stake for their own land. Because of the initial conflict with Curley and the awkwardness of Curley's wife, Lennie wants to leave right away. Lennie says to George, "This ain't no good place. I wanna get outa here." Once Candy joins in with George and Lennie's dream of owning their own land, George decides that in one month they'll have enough money. At that point Lennie and Candy can go ahead, and George will stay behind to earn the remainder of the money. Unfortunately, they never get their own land.
According to George, Lennie and George can leave the ranch when they have made enough money, which is $800.
Lennie wants to leave as soon as possible because he wants to tend the rabbits, as George promised him. (It was never really revealed exactly when Lennie wanted to leave.)