The Catechism in Questions & Answers

70. In what span of time did God create the world?

God created the world in six “days of creation”. The term “day of creation” refers to a period of time whose duration is not precisely specified. A “day” in the creation of God is not to be equated with a day according to our reckoning of time.
Genesis 2: 2 states: “And on the seventh day God ended His work which He had done, and He rested on the seventh day from all His works which He had done.”

“[...] that with the Lord one day is as a thousand years, and a thousand years as one day.”
2 Peter 3: 8 (excerpt)
“For a thousand years in Your sight are like yesterday when it is past.”
Psalm 90: 4