The process of dying from old age is a very slow one in general, taking anywhere from several hours to several days. I wouldn't mind going that way, if I was old I would more likely accept my passage through life.
It really depends on what's killing me. If I'm being murdered, I want it to be fast. If its disease, I want it to be slow so there is still a chance of survival.
If I was going to die in a peaceful way like from old age then slow but if I was going to be tortured to death then fast so I don't have to feel all the pain.