The Bible does not say. It does say that "One day with the Lord is AS a thousand years, and a thousand years AS one day". This is a simile. That is backed up by the Psalm that adds that a day to God "is AS a watch in the night" - 3 or 4 hours. Another simile. Illustrations are being used to help us Earth-bound, time-bound people grasp that for he who dwells in eternity, 24-hour Earth days are not the dimension he operates in. Yes, he created our planet to be ruled by night then day, with all the seasons by which we measure years, but the Creation account is unique. For a start, the seasons did not kick in until Day Four (Gen.1:14). For those who think that Day was a 24-hour Earth day as we presently know it, gravity would have the unformed, dark, void Earth spinning round far faster, meaning complete revolutions of about 5 hours. The gravitational influence of the moon helped slow the rotations down, but in the future, the moon will slowly move further away from Earth, making its revolutions gradually longer.
Here is what Professor Bob White, Geophysicist, (and Christian) says in answer to the question 'Did God do all this within six 24-hour days?':
"The six days are a literary device of a week's hard work. In any
case, as a geologist I find it amusing to reflect that when the Earth
was created it was spinning much faster than it does now, so one day
lasted only about five hours. Due to the gravitational pull of the
moon and the tides, which absorb energy, the Earth's rotation has been
slowing down ever since, so actually six 24-hour periods wouldn't fit
either!" God, the Big Bang & bunsen burning issues, Ed. Nigel Bovey, pp. 139-151 for Prof. White's full chapter. (Authentic 2008)
We now know from lasers (one planted on the moon) that over thousands of years interacting planets slow down. The moon's orbit continues to vary because Earth's rotation is slowing it down due to 'tidal braking'. Tidal bulges interact with the Moon's gravitation to produce a decelerating torque that slows the Earth's rotation, causing an increase in the length of the day.
The equal and opposite reaction is a torque exerted by the Earth on the Moon's orbit that increases its angular momentum. This is achieved by an increase in the distance between the Moon from the Earth, pushing it 1.25 inches farther away each year, and a decrease in the rotation rate of the Moon about the Earth, which increases the length of the month.
The Earth's rotation decelerates more rapidly than that of the Moon, so eventually the angular velocities of the Earth and the Moon will be equal, a condition called synchronous rotation. At that point, a billion or so years hence [assuming God has not burned this Earth with fire, replacing it with a new Earth], an Earth day and month will be equal, at about 47 current Earth days, and the Moon's distance from the Earth will be 135 per cent of its current value. Then, for millions of years, one side of the Earth will face the Moon. Half the world will see the Moon, while the other half will never see it.
The Sun's tidal pull on Earth is only half as strong as the Moon's, so as the Moon departs, the Sun will grow relatively more influential. Ultimately, it will make Earth spin even slower, and the Moon will start falling toward us. It will break apart before it reaches 10,000 miles away because its silicate rocks are only half as dense as the Earth's heavier materials. Of course, God is sovereign, and we know He has promised that the present Earth and Heavens will be replaced with "a new Heavens and a new Earth, in which righteousness will dwell." (2 Peter 3:10-13)
When we look at the different meanings of the ancient Hebrew word for 'day', we see that the six days of creation need not be the usual meaning of a 24-hour Earth day. In Gen. 2:4 (KJV) all six 'days' are called "in the day that the Lord God made the earth and the heavens." The six days become one day. And in verse 17 God warns Adam that "in the day you eat" (of the forbidden fruit) "you will surely die". But we know Adam lived for 930 years. Clearly a 'day' has various meanings. The Bible states that one day is with the Lord as a thousand years - that seems to fit Adam's dying before he reached 1,000 years - a day in God's sight.
When we read the Genesis days as being epochs - eras - then there is no problem. After all, God is known from what the Bible says as being very patient and not in any rush to do things. He allowed 400 years to pass before bringing judgment on the Amorites (Gen. 15:16; 1 Sam 15:2). He allowed thousands of years before the promised Messiah was born. Nearly two thousand years have now passed and still we await Christ's return but, from God's point of view, that could just be 2 days!
Back to various interpretations: the idea then arose that the seventh day (of rest) has to be on-going even till now, because the Bible speaks of people being able to enter into God's rest, and that that day continues - see Hebrews chapter 4. It had been 4,250 years since creation (according to young Earth creationists) when Hebrews was written. Now they would say its 6,190 years, and God's day of rest still has not ended. This indicates to many that the seventh day could be 7,000 years in length (making Earth 42,000 years old by the start of Day Seven).
Then there are creationists who believe in an old Earth - that the universe was created by God first, and it took however long it took for the Earth to be formed to get to the start of the Genesis description of it being dark, void, and covered in water. Then God started with bringing light into the darkness, order out of chaos, and land out of water. Perhaps when God said, "Let there be light" (on Day One) that happened suddenly, in a moment. Or perhaps the language incorporates billions of years for the myriad forms of light to be in place before visible light then erupted when the first suns burst into incandescent visible light. Note that light that is visible to human eyes is but one form of light. Check this link - http://study.com/academy/lesson/light-waves-definition-types-uses.html
That is a brief summary of various interpretation. I'm not coming 'down' on any side. This is for your consideration, to show the difficulties involved in answering your question. The Genesis account may be deliberately short on details because nobody's salvation depends on knowing how long each day of creation lasted. We are called to believe that "In the beginning God created the heavens and the earth" and that all life on our planet was created by God, with Adam deliberately formed perfectly, in the image of God, by God's hand. We are not called to put dates on any of that, or to know HOW God created - just that He did.