Old friend of mine used to fly SLF until he upgraded to an airline only flying boxes. He would do 2, and only 2, PA's. The first would be the usual "welcome onboard, flight time is 11 hours, sit back and relax yada yada." and the other would be made at the half-way point and consist only of "Half-way there".

It's a service to the people down the back, a service which may or may not be executed depending on a large number of factors.