Greater Spell Focus (Conjuration) does not affect summons' duration, as it should.

Where does it say it should?

It has unofficially been stated a few times in the recent past by a number of DMs, but it didn't use to be the case.

I think this was posted a long time back as part of an overhaul to all the spell focus 'special' effects. Many of the changes never went in while some did, though. At least I think that's how I remember it.