Durable Spells is a skill available in all spell schools. It requires 25 skill in the the school you want it in.

It will increase the duration of the spell you use by an amount depending on the spell you are casting, the skill in that spell and your skill in Durable Spells in that school.
Your spells linked to Durable Spells will have the same skill level as your Durable Spells.
For the list of linked spells see Linked Spells

Magic Schools
Regeneration MagicRaw MagicInfliction MagicAir MagicEarth MagicFire MagicWater MagicSpell ChantingArcane MagicWitchcraftNecromancy
Magic skills
Durable SpellsMana EfficiencyQuicken SpellsIntensify Spells
Magic Global Skills
Veteran MageMana ConservationArchmageReagent Conservation
Linked Spells