For about how many years of geologic time have humans existed on earth

For about 4.5 million years, and still exist on earth

Explanation:

Humans are the most highly complex organisms and are comprised of well-developed brains. It shows advanced characteristics and is well developed social animals that can adapt well in any environment.

The humans are one of the major God's creation which has played and still playing a great role on earth.

These human beings were first appeared on earth about 4.5 million years back, during the time of Pliocene. These were the Hominoids and Australopithecus, from which they have evolved and became more advanced. They represent on earth since the Pliocene and still exist on earth.

Thus, it can be estimated that they are found to be present for about 4.5 million in the geological time scale and is the major and dominant species on earth now.