The Dungeon Master and the Player never wants to track 1 hour of Mage Armor. Giving spells time limitations like this is frustrating. Just give spells an encounter duration not a 15 minute or 1 hour duration. Mage Armor is a O level spell so it is At-Will according to the rules. And just assume that the first thing the caster does when going into combat, is to cast Mage Armor.
Why does Mage Armor add the caster's dexterity modifier to his armor class? A caster is likely to have a lower dexterity than his intelligence. Mage Armor should add the intelligence modifier which is the primary ability of a caster to his armor class. Intelligence Mod = magic mod.
So, just increase the caster's armor class permanently by adding the intelligence modifier and be done with it.
Why does Mage Armor add the caster's dexterity modifier to his armor class? A caster is likely to have a lower dexterity than his intelligence. Mage Armor should add the intelligence modifier which is the primary ability of a caster to his armor class. Intelligence Mod = magic mod.
So, just increase the caster's armor class permanently by adding the intelligence modifier and be done with it.