Since 2000's X-Men, the Stan Lee cameo has been a Marvel movie tradition. In the past 13 years, he's been in just about every one of them, with a few notable exceptions. Sadly, it seems that the upcoming X-Men: Days of Future Past will be one of these exceptions.
Recently, Stan Lee participated in a panel at Fan Expo. Bleeding Cool reported that Lee confirmed he would not be available to shoot a cameo in Days of Future Past. Apparently, Lee had to choose between attending the convention or shooting the cameo, and he "couldn’t disappoint the fans so came out to Fan Expo instead."
Lee also did not make an appearance in The Wolverine, which premiered earlier this year. He confirmed it was because the shooting location in Australia was too far for him to travel.
It's unfortunate news, but at the very least we'll be seeing him in Thor later this year, as well as possible appearances in Captain America 2 and The Amazing Spider-Man 2 in 2014. And while we're at it, why not watch all of Stan Lee's cameos to date, from X-Men through The Amazing Spider-Man: