I was part of a discussion a few weeks back where we talked about that. The general agreement was that lengths can vary greatly, even on the same plane/seat.

As an example, I was on a 763 both ways between ORD/DUB in October, in seat 21H on both segments. 1 direction I had to have the extender, the other I had plenty of slack.

There's absolutely no consistency on length of the belts.