When you are changed by magic, your skin also toughens.
Prerequisite(s): Con 13.
Benefit(s): Whenever a transmutation spell or spell-like ability affects you, your natural armor bonus increases by 1. If you have no natural armor bonus to Armor Class, treat your natural armor bonus as 0 for the purposes of this feat. The bonus to your natural armor bonus increases by 2 if the caster level of the effect is 10th or higher.
The duration is not noted in the source text but since the feat specifically says “whenever a transmutation spell or spell-like ability affects you…” it can be assumed the duration is as long as the transmutation spell continues to affect you.
Pathfinder Player Companion: Magic Tactics Toolbox © 2016, Paizo Inc.; Authors: Alexander Augunas, Steven T. Helt, Thurston Hillman, and Ron Lundeen.