So there are varying degrees of overtraining. Most people simply aren't overtrained. They are just in stale workouts where there is no underlying structure other than a split, exercise selection, and they go in and work hard in a given rep range. A well structured program will bring an athlete right to the point of overtraining (this point is called over-reaching). Training beyond that point is suboptimal (like in the 5x5 if it's properly setup most people can't make true record lifts beyond 2 weeks - it's actually closer to 10 days but there are some freaks who can handle 3). If pushed hard beyond this point (and we are talking record lifts here with the weights set exactly right which tends not to happen without experience or coaching) their lifts begin to drop the following week and it takes a more aggressive or lengthy deloading period to compensate. Ideally, like a race car driver you either want to be hard on the gas or hard on the brakes at all times. Overtraining oneself is like making a shitty turn and having to nurse it through - just suboptimal. Now a single week isn't so bad (you still don't see this even before competition though so consider that) but a 2-3 week period as we originally spoke about just doesn't have a good place other than a mental vacation - maybe after a major contest and before beginning a new macrocycle in preparing for the next year's championships.
So when are longer periods appropriate. Well, massive overtraining which most people on this board probably have never experienced (you have to be fairly dumb to ignore the warning signs and the warning signs that this is coming). I did this once as an expiriment and although I don't have a lot of familiarity with optimally recovering from a purposely created horridly overtrained state I did elect to take 2 weeks off and then 2 weeks of reacclimation work (basically more deloading and light lifting) before I was ready to push hard again. Basically, CNS burnout. I got nothing from the 8 weeks of loading (yeah - it was on purpose) and came back even a bit weaker than when I began simply due to the detraining.
Basically, there is an ideal window of over-reaching and deloading that keeps you moving at optimum velocity. In peaking for a major compeitition, the goal is to be 100% on the day so forward velocity is not the issue but merely being at one's best on a single day after which training will recommence from the ground up. So is 1 complete week off bad - not really. Is it suboptimal - chances are yes but not by a massive margin. Is 2-3 weeks off bad - yes unless you are massively overtrained or simply in need of a vacation. Is it suboptimal to a current ongoing program - yes, by a very large margin.
Do hopefully you guys get the feeling that what we are talking about is balancing load and recovery here to meet a specific goal. It's a bit of an art as well as a science. This is why it's very hard to perscribe much specificity and I always stress learning one's tolerances. This knowledge enables you to optimize your loading cycles so that you don't over or under load (gas) that way a standard deloading period (brake) is all that is required to get you ready to push hard again (gas). Long and rambling but that's the jist.