I forget exactly how long it lasts (I think it was 60 seconds), but it certainly lasted long enough to warrant the Talent Gauge usage (unlike say, Speed in most situations). I believe it starts at a 40% Def/Eth Def buff for each party member, and increases by 5% each time you upgrade it, making it 85% at max level. Armour is just amazing, especially since it only takes 2 easy quests to get.