It can't be quantified in an absolute sense since people recover at different rates.
Short term, day's (i.e. if you train hard for a few days on the trot, you will become tired, ache, performance might drop, but you are not overtrained), maybe even a few weeks (similar to with the days scenario, a training block lasting several weeks can leave you feeling pretty bad too).
Medium to long term would likely be several weeks to months.
A distinct difference between overtraining and over reaching is seeing what happens when you ease back, if you train properly (for you) after a hard block (in which you may experience some things that lots of people think mean you are overtrained) when you ease up for a week or two, you will recover and performance will improve beyond what it was previously. Typically with over training it will not, the recovery will take a disproportionate amount of time and fitness will not increase.
If you are interested do a little reading on training periodization.