Intensity, can also change the number of sets needed it's all how much work your muscles do. You know when your coping out or not, the decision is in your head.
If your going all out, you don't neccessarly need lots of time or sets. Of course all out also means no cheating and keeping weights you can handle with proper form. Each set you should be putting in more and more intensity until nothings left.
personally I can trash most muscles in about 1hr (lower) -1/2 hr (upper)