This is a good question. There are three places in the book that mention Curley's marriage. In chapter two, there are two small reference.
First, the guys say that Curley has gotten more arrogant since he has gotten married a few weeks ago. None of the men respect Curley or like him. They like him less now that he has gotten married.
Second, as the men are playing cards, they mention that Curley's wife has got the "eye". By this they mean that she is checking out other men; she obviously does not like Curley either. In particular the men believe that she is checking out Slim. They also mention that Curley and she have only been married for two weeks. This might be an estimate, but it is our closest guess.
In conclusion, Curley and his wife has been married for a very short period of time, probably two weeks.
Here is a quote:
“Well—she got the eye."
“Yeah? Married two weeks and got the eye? Maybe that’s why Curley’s pants is full of ants."
“I seen her give Slim the eye. Slim’s a jerkline skinner. Hell of a nice fella.
Slim don’t need to wear no high-heeled boots on a grain team. I seen her give Slim the eye. Curley never seen it. An’ I seen her give Carlson the eye."
George pretended a lack of interest. “Looks like we was gonna have fun."
The swamper stood up from his box. “Know what I think?” George did not answer. “Well, I think Curley’s married . . . . a tart.”