After burning Mrs.Blake's house, Montag goes home very upset, and is lying in bed next to Mildred, trying to remember when they met. He can't, and neither can she. To justify her lack of memory, she states, "It's been so long," but Montag responds with, "Only ten years, that's all, only ten!" So, they either were married ten years ago, or first met ten years ago, and that is the time-frame that we are given in the book. They both conclude that when or how they met "doesn't matter" but Montag is still disturbed. He realizes that, despite being together for ten years, Montag feels that "she was so strange he couldn't believe he knew her at all." Even more sad was that "if she died, he was certain that he wouldn't cry." This alienation and indifference is, unfortunately, typical in their society, and one of the things that prompts Montag to try to find answers.
I believe its 10 yrs.