A long time ago is a correct, though not very satisfying answer. Unknowable is also a correct answer, but the Bible, genetic and archaelogical records provide some clues.
Let’s begin with what the Bible says. A literal reading of the genealogy as recorded in the book of Genesis would put Adam at about 6000 years ago (4000 BC). However, the Hebrew word for ‘Son of’ or ‘Bar’, later ‘Ben’, also means grandson of, great-grandson of, etc., so there could be very many generations between each person identified as ‘son of’ in the book of Genesis. Just as in later instances, only the noteworthy or significant individuals are identified.
We also know Adam possessed the ability to speak, communicate, had language, intelligence, etc. He also possessed a ‘God Given Spirit’ in addition to his DNA as a Homo Sapien.
Archaeologists have dated civilizations with language, tools and agriculture back to 10,000 BC, or 12,000 years ago. If we accept the Biblical account of God breathing his spirit into mankind at the beginning, this would put the first Adam possibly as far back as 20-40,000 years ago. Some geneticists claim that humans, as we know them, have been around much longer. For a better understanding of this topic, we encourage you to browse the Reasons to Believe website and consider the scientific work of Dr. Hugh Ross and his team who have done an outstanding job confirming the Bible and Science.
While wanting to know the date of the creation of Adam and Eve, for all of us living today, a much more important question is: What happens when I die?