How long have Mildred and Montag been married in the book "Fahrenhiet 451"?
We're not told specifically how long Mildred and Montag have been married, but are led to believe they met ten years ago and married not long after that. We know from one of Montag's early conversations in the book that he became a firefighter ten years ago, at the age of 20. Therefore, we can assume he met and married Mildred at about the same time he started his career.
However, the significant point is that neither he nor Mildred can remember when they met or married. Since reading is banned, there would likely be no written record. This underlines the importance of having written records, because memories can fail. There's also a strong sense in this passage that taking pills, as Mildred does to cope with the emptiness of her life, leads to memory loss. The couple's inability to remember their own past becomes a symbol of an entire culture losing its history. Montag thinks the following as he lies in bed:
He held both hands over his eyes and applied a steady pressure there as if to crush memory into place. It was suddenly more important than any other thing in a life-time that he knew where he had met Mildred.
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.