Leaving at "Amen" or hanging around to fellowship- that is just another thing that
they will tell you "officially" not to judge others about, but, in reality, will highly
gossip and criticize anyone about if they don't stay awhile.
The 15 minutes is officially for providing a lengthy enough gap between congregations
using the same hall to allow sufficient time for gathering literature and having a
meeting for field circus. It is universally applied for uniformity of all public talks- it just
has the added benefit of forcing the speaker to stick more strictly to the outline.
I believe it is in preparation for shutting down some kingdom halls and forcing more
congregations into the same hall. If they kept Saturday evening free, every single
auditorium could house 5 congregations.