Guild buffs are very neat bonuses. However, they only last an hour long. This of course makes for an unneeded time sink in running back to your airship and getting rebuffed in between quests.
Adding to the duration of these buffs will not really make them more powerful except in the rare cases where dungeons last over one hour.
So my suggestion is simple. Make all guild buffs last six hours long. This should be long enough to last through any quest and eliminate most of jogging back and forth that these buffs otherwise generate.