This isn't too surprising as he did not appear in X2, X-Men: First Class, and The Wolverine. It seems that X-Men films are where he is missing the most. Some are guessing his age has something to do with it because the man will turn 91 this year, but he insists that he is simply choosing what he wants and doesn't want to do.
The complete list of Stan Lee cameos include:
X-Men: The Last Stand
The Amazing Spider-Man
The Incredible Hulk
Iron Man 2
Iron Man 3
Fantastic Four: Rise of the Silver Surfer
Captain America: The First Avenger
Now, keep in mind that just because he passed up Days of Future Past doesn't mean he won't appear in any more Marvel films. I have no doubt he's somewhere to be found in the Phase Two movies and/or The Amazing Spider-Man 2.