"And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years" (John 6.3).

By adding up the ages of the Bible you come to 4004 BC. Adam was 18 years old 3986 BC. There are 120 Jubilees from 3986 BC to 2015 AD. That's 50 x 120 = 6000 years.

God is saying He can get this all done in 6000 years.