How long was the first day in genesis?

The term "day" when used in the Genesis creation account refers to an extended period of time, most likely a period of thousands of years. That same term context is used in our modern times, such as when people say "back in the day", or "during my grandparents' day", etc.; People aren't referencing a specific 24 hour period, but a passage of years during a certain era. How do we know that this is the case in Genesis though?

The 1st thing to consider is how old the Earth actually is. Science estimates it's millions of years old, and if you look at the start of Genesis, the Earth is already there before the 1st creative 'day' began. (Genesis 1:1) Next, with all that Jehovah God had done in just that 1st period of time, He took longer than 24 hours dividing the light "from the darkness". Evening was created as well as morning, signifying the start and end to the 1st period of time. (Genesis 1:4-5) Same with the following stages of creation as the 1st chapter draws on. (Genesis 1:6-31) To drive the point home however, comes from the 7th "day", when Jehovah rested from His earthly works. (Genesis 2:2) Thousands of years later, the Apostle Paul wrote that they were still living in His "rest day" at that time (Hebrews 4:3-10), showing that the term "day" is not always 24 hours, but sometimes an extended length of time.

(New World Translation of the Holy Scriptures)