Increased Buff duration only works on buffs that don't need an increased buff duration. So basically that affix is a trap and completely useless. Don't invest into it. As for why: I assume it's because of balance reasons, though that's not a valid reason anymore ever since sacred armor was reworked (it used to make you invulnerable. Combined with increased buff duration and cooldown reduction you were able to be permanently invulnerable).
So yeah, I'm pretty sure that affix will be removed at some point because the way it is now, it's completely useless.
Ok, thanks. It seems like that's the kind of information that should be readily available so someone doesn't say MO all of their jewelry expecting it to work .