In Gen 5 each berry reduced 10 EVs. Before that, the first berry reduced to 100, then 10 after that.
Note: This question has been asked before, but not with Gen 6.
Same thing. But with the new Super Training feature, using "Core Training" will sometimes allow you to find a Reset Bag, which removes the EVs of all stats of a Pokemon. Wayyyy easier than using the Berries now.