The Extend metamagic feat will double the duration of a spell. Can you use two Extend feats to quadruple the duration?
While on the subject, is there any other way to stretch out the duration of a spell (other than Permanency)?
You can't apply it multiple times to the same spell.
Multiple Metamagic Feats on a Spell: A spellcaster can apply multiple metamagic feats to a single spell. Changes to its level are cumulative. You can't apply the same metamagic feat more than once to a single spell.
As for other ways to extend duration, the only ones that come to mind are the Spell Tinkerer Arcanist Exploit and a Ring of Continuation. There probably are others.