When Kinetic Armor is renewed, its new length is determined by your current enchantment modifiers. For example, if you cast Kinetic Armor with no enchantment modifiers, then it will be active for 8 seconds. If you then switch to a weapon with +20% enchantment length and cast a spell, Kinetic Armor will be renewed for 10 seconds.
Ideally, Kinetic Armor can be held up with at least one quick recharge, low cost spell, such as Stone Daggers.
Earth Magic has relatively few low-recharge spammable spells, so, because of its strong effect even with a modest investment in Earth Magic, this spell could be more useful with an attribute with more quick spells, such as Air or Fire Magic. Searing Flames is especially useful, because it will almost always be spammed.