From uesp:


Duration spells do not have a pre-determined duration. Rather, the duration is determined by how long you hold the casting trigger.


So if I hold casting trigger a very lOng time my oak flesh will last longer?

That's only "concentration" spells like the novice destruction spells or the healing spell that everybody starts off with. Alteration spells like oakskin DO have a set duration that can be extended by the Dual-casting or Stability perks in the Alteration tree.
-Loth

Edit: But there are several spells in Alteration that DO depend on holding the trigger: Detect Life, Detect Dead, Telekinesis. You can tell the difference in the spell description if it gives the mana cost as "X mana per second".

Oh holding the trigger AFTER u cast! I thought before.


I get the healing, flames thing. They worded that weird.

Yes, they did.
-Loth

for some spells you have to hold down the trigger to cast them too.. like soul trap and candlelight.