your body adapts very quickly to the stress you are placing on it (weightlifting) so in order to avoid stagmenting in your routine you want to keep your body from adapting to what you are doing. If you did the same exerciese with the same weight you would never progress any further, hence progressive resistence weight training. Varying up your training doesn't necessarily mean changing exercises although that can also be helpful. You can also change little things like the tempo of an exercise, rest time, rep range, volume and intensity and so on. There is no set time frame fro a BB to change his routine by and if he doesn't then he won't be successful. For instance the squat is in my leg routine year round, but I add things and ways to keep it fresh. You can put in time with an exercise and still vary things up as well. I hope this helps answer your question.