How many years was the Old Stone Age—before the transition to the Middle Stone Age?

It is generally agreed by historians that the Old Stone Age—also known as the Paleolithic period—ranged from about 2,600,000 BC to 12,000 BC, when it was replaced by what we now know as the Mesolithic period (i.e., the Middle Stone Age). The Middle Stone Age lasted for approximately 3,000 years until the dawn of agriculture.
As the name implies, the Stone Age was characterized—mainly, but not exclusively—by its use of stone implements. Because the Stone Age is named after technological developments rather than actual date ranges, its length varies in different parts of the world.
Much the same could be said of the ending of the Middle Stone Age, as agriculture developed at different times in different places throughout the world. During the Mesolithic era, the earth's temperature increased dramatically, leading to the large-scale melting of ice sheets as well as a rise in sea-levels. Across the globe, the shape of land masses also experienced enormous change, with the Japanese islands breaking free from mainland Asia and what is now Great Britain separating from the European continent.
The flooding of vast swathes of territory—caused by rising sea-levels—resulted in major changes in land-use worldwide, leading to the development of agriculture. This development then heralded the end of the Middle Stone Age.