That was actually my biggest disappointment with Gwyn, the fact that you could literally parry him to death. And he is so aggressive that it kinda difficult to miss a parry opportunity. In total it probably lasted about 1:30 for me. I feel like once you dealt with Smough and Ornstein it was kinda downhill as far as the boss battles go.
That being said though, NG+ Four Kings fucked my life up. I was unprepared for the buffs they got.