When you've learned more about astrophysics from applying them in Orbiter than reading them somewhere.
When both your desktop and school laptop have lots of Orbiter stuff on them!
When you learned about C++ from the Orbiter-Forum and applied the concepts you learned to your Java class and made an A on your midterm grade!
When you want to install a bookcase behind your desk chair so you can turn around like the guys in Mission Control do to reach for notebooks and suuch.
When you change your ham radio callsign so you can say Delta Glider for the phonetics (example, KG4DG is kilo golf four delta glider) [I haven't done this...yet!]
When you try to fly a 747 in FS2004 with numpad-2,8,4,6,1,3!