After few tests here it is what I've discovered: - The bard mastery bonus doubled is NOT the mastery spells effect, but this one: - When the effect is active, the armor have a lighter color. - The cooldown vary from 30 minutes (1 piece) to 15 minutes(full set) and EACH PIECE has the cooldown. So you can activate the effect 4 times in a row! - There is no buff icon when the effect is active. - The effect can't be activate if you have no mastery. - The cooldown remaining is visible on each piece when the effect is over.