It can last anywhere from 6 minutes to 10, but usually it is about 8.
The duration of any eclipse depends on the exact geometry between the Earth, Moon and Sun, and particularly the distance between the Earth and the Moon.
But there is a Blue Moon.
How long did the July 20th eclipse last?
So, in order for the umbra to touch the Earth, it must be closer than the average Earth-moon distance.
The fact that an eclipse can occur at all is a fluke of celestial mechanics and time.
How Solar Eclipses Work:
This is caused by diffraction, a property of light.
And because sunlight is blocked during a total eclipse, some of the brighter stars and planets can be observed in the darkened sky.