In what part of the country and in what time period does the novella take place?

Steinbeck opens Of Mice and Men with a detailed description of the location:

A few miles south of Soledad, the Salinas River drops in close to the hillside bank and runs deep and green. The water is warm too, for it has slipped twinkling over the yellow sands in the sunlight before reaching the narrow pool. On one side of the river the golden foothill slopes curve up to the strong and rocky Gabilan Mountains...

Soledad is a city in Monterey County in the central region of California. With its warm but not hot weather, it is most famous for its agriculture. Many people flocked to the region during the Great Depression (1929–1939) in a search for work.
Lennie and George find work at a ranch only a quarter of a mile from where they are at the beginning of story. George says to get there, they just had to "follow the river."
It's worth remember that Soledad is Spanish for solitude. This is more in reference to the surrounding area—made up of mostly farmland—than the city it itself, and there is a suggestion that the migrant workers go to Soledad as an outlet when they get lonely or bored. As Crook unkindly tells Lennie;

An' where's George now? In town in a whore house. That's where your money's goin'. Jesus, I seen it happen too many times. I seen too many guys with land in their head. They never get none under their hand.

Of Mice and Men, by John Steinbeck, takes place on a ranch in California. We know this because the two main characters, Lennie Small and George Milton, are migrant workers who are in search of jobs. This story takes place during the Great Depression, which means it occurs sometime in the 1930s.