An Assassin with 8 Critical Strikes and 9 Dagger Mastery has about a 21% chance of landing a critical hit, which is more than 1 in 5. With 13 Critical Strikes and 12 Dagger Mastery, that figure raises to 30%, assuming one attacks a level 20 foe. Simply attacking with the IAS of 33% should be more than enough to refresh this enchantment. See the Critical hit article for more information.
Gaining +24 armor is equal to gaining +34% reduction to all damage that is affected by armor.
Beware Soul Barbs or other such anti-enchantment hexes while under the effects of this enchantment, as each critical hit will count as a reapplication of the enchantment, and will trigger the effects of the hex.