there are "caps" when you get additional dot ticks as affliction, but they dont really affect your damage in pvp.
for people who still wanna know:
UA: 7,16% (872rating); 21,4%(6637rating)
Corruption: 5,57% (231rating); 16,66% (4717rating); 27,76% (9211rating)
Agony: 12,52% (3043rating); 20,81%(6400rating); 29,16% (9778rating)
every number +5% if youre not playing with a class that provides spriest or moonkin aura