As Orbiter is a simulator and project made on Dr. Schweiger's freetime for free, there's no regular upgrades or planned releases. He does his work when he finds some time to do it.
The last release was in 2010 and the one before that was in 2006, so if it follows the pattern, you could "expect" it in 2014, but as mentioned: nothing is certain.
But of course, I don't know for sure. I guess Dr. Martin Schweiger is the only one with a good answer.