Overtraining occurs when one trains too often, too long, too hard or any combination of these. Plenty of other factors in life can contribute as well such as lack of sleep, poor nutrition, and stress. Overtraining appears to build up slowly and can go unnoticed. When I first began training I was hell-bent on increasing the weights I lifted. I thought I was improving, but I was fooling myself. I’d change the cadence a bit, take an extra second rest between reps, ever so slightly decrease the range of motion, subtly cheat your way through the sticking point a ¼ second in order to complete the rep and voila I improved. On paper I continued to improve but I was gradually becoming increasingly over-trained. The end result is usually burn-out, injury or sickness. I had my share of that.
From what I have observed it take weeks to build up overtraining and weeks for it to dissipate. Example One: