According to a trainer I had a few years ago, optimum results come when you vary the set, rep, and tempo every 4-6 weeks. Even that is very general. There are certain tests that can be performed to determine what works best for you, and certain muscle groups respond better to some protocols than others. The best book I've seen on this is The Poliquin Principles. That's what my trainer had recommended to me when I had the time to actually read up on all this stuff. Nowadays, I just hit him up for new programs every few months.