Strictly speaking, the Bible doesn't say how many of thousands of years the Earth is.
Various scholars, since early Christian period, have tried to determine the date of Adam's creation by using the figures given in the bible. But the dates can vary, depending which texts or translations a person may use. For example, the dates given in Masoretic Texts (used in KJV and other modern English translations) have different numbers of years (in the Genesis) to the Greek Septuagint Bible.
I don't know which person or people given this date, but a lot of people favored 6007 years ago, or 3996 BC.
According to the Hebrew calendar, the traditional dating for current year (2010), is 5770 AM (Anno Mundi), which is 3760 BC. This year WILL BE 5771 AM, in September 28th (Jewish New Year).