Stand Off Time (Planes In The Gates)

Several months ago it was a big discussion on AVSIM regarding stand off time period. Most people came to the conclusion that 30 minutes is a minimum time which is required for AI Plane in order not to dissapear.I did some experimenting and it seems like something is wrong.I've spotted default VHHH during 7 or 8 hours (in 4x acceleration mode). Most of the planes (767,747, 777) in VHHH have a stand of period about 90-120 minutes. However, even after this huge period time most of them dissapeared. Sometimes, in 3rd party scenery (WSSS and RJAA) planes are dissapering as well but they appearing again in the gates after 2 or 3 minutes. What's matter?Is it a FS bug?Is it a bug in a certain .bgl file (or bug for certain scenery like VHHH)Should I increase stand off time for huge jets up to 3 hrs? (I won't be easy of course)Your comments would be much appreciated.Once again, sorry for my bad English.