I hate how short the buffs spells lasts in the game so I made them last longer. I tried to keep the duration reasonable and to prevent spells from being overpowered. Thank you for the endorsements and I hope you enjoy the mod!

Version 2.1 notes: Fixed the issue with doubled/tripled casting cost. Everything else from version 2.0 remained the same.
*note The text strings of the Spell Books themselves have NOT been edited, so the books will still have the default casting time. Ignore it.
**note Conjure Dremora Lord and Rally says "per sec" on them. "Per sec" is not suppose to be there, they cast normally. Ignore it.

Version 2 notes: I redid the .esp file completely, spells no longer cost double or triple to cast. Fear is back to its default time, Conjuration spells have been added, Waterbreathing and Rally has been added.

Version 2:
Conjuration

Changed to 3 minutes:
Bound Battle Axe
Bound Bow
Bound Sword

Changed to 2 minutes:
Conjure Dremora Lord
Conjure Dragon Priest


Alteration
Changed to 3 minutes:
Candlelight
Magelight
Oakflesh
Stoneflesh
Ironflesh
Ebonyflesh
Dragonhide

Changed to 2 minutes:
Waterbreathing

Illusion
Changed to 2 minutes:
Courage
Rally

Version 1:
Candlelight duration x 3
Oakflesh duration x 3
Magelight duration x 3
Stoneflesh duration x 3
Ironflesh duration x 3
Dragonhide duration x 3
Courage duration x 2
Fear duration x 2

Installed via NMM (recommended) or extract the rar, move LongerSpellDuration.esp to the Data folder in where your Skyrim directory it's installed. Don't forget to tick the .esp file in Data Files of your Skyrim Launcher.