The quick answer is:  a few million years.


Ninety miles downriver from the popular viewpoints around Grand Canyon Village in Grand Canyon National Park on the South Rim of the canyon is the Uinkaret volcanic field.  Lava flows from this field have cascaded down into the Grand Canyon, damming the Colorado River.  Most of the lava has been swept away by the continued flow of the river.


The dams ranged from several hundred to more than a thousand feet in height.  The largest one created a large lake that extended more than two hundred miles upriver, all the way to Moab, Utah.  At its highest point, the lake reached up to the level of the Redwall Limestone, halfway up the walls of the Grand Canyon.  These lakes were short-lived, probably not playing a major role in controlling the flow of the Colorado River for more than a few centuries.  It is the age of these lava flows that is the key to understanding when the Grand Canyon reached its current appearance.


Some of the lava still sits in the river.  A dark angular rock known as Vulcan’s Anvil formed from one of the flows and is about a mile upriver from Lava Falls.  When this and similar rocks of lava  erupted and cascaded down the canyon walls, the Grand Canyon was about as deep at that time as it is today.  The age of these lava flows is about one million years.  By then, the canyon that we see today had formed.


But how much older might the Grand Canyon be?  That is, when did most of the digging of the Grand Canyon happen?  Those questions are answered by looking elsewhere in the Grand Canyon and the surrounding area and determining the time of erosion.  Such work shows that the two end segments, Marble Canyon to the east and the westernmost segment of the Grand Canyon, were both carved in the last six million years.  That is when the Grand Canyon that is seen today began to form.  But there is a caveat.


There is evidence of a much older canyon.  The middle section of the Grand Canyon, known as the Hurricane Segment, was deeply eroded to near-modern depths by fifty-five million years ago.  It was carved by what was one of a series of rivers that once crossed this part of the continent.  The various cuts and fills created by these rivers were later integrated the drainage system of the modern Colorado River, which, in the last six million years, widened and deepened the canyon and, by a million years ago, gave it the appearance that it has today.