A new skill for casters. "Through tireless hours perfecting your casting the length of time buffs remain active is increased!"

Maybe capped at 30 minutes or however you want to balance it? Perhaps a softcap on rank differences so lower rankers cannot say do a whole lab on a warlord sorc spells.

Not sure I am too keen on this one, at least not as a permanent skill.

Maybe as a special event skill.

Not very useful when your spells cost nothing and you have unlimited mana.