In The Great Gatsby, how long had Myrtle and George been married?

In Chapter VIII of The Great Gatsby, after Myrtle's death in the previous chapter, we hear of what happened after Myrtle's body was taken away, from the testimony of Michaelis, who is a neighbor of the Wilsons. This testimony is given to us via Nick. 

Michaelis runs a "coffee joint" (143) near the ashheaps, and while he does not seem to be exactly a friend to the Wilsons, he does try to help George after Myrtle is killed. He asks George how long they had been married and whether they had any children, and this is when we learn that George and Myrtle had been married for twelve years and had no children.  Michaelis offers to go find a priest for George, someone to talk to and counsel him in his grief.  George declines this offer, and as the conversation continues, Michaelis learns that George had suspicions about his wife, that she had a lover, and then George makes the accusation that her lover has deliberately run Myrtle down with his car.  It is this misidentification and misunderstanding that results in Gatsby's tragic death, since it was not Gatsby who was driving the car nor who was Myrtle's lover.