Constructing a timeline of the ancient history of the galaxy from the movies and statements from Lucas

“Over a thousand generations” before the movies: The Jedi were founded on Ach-to.

“Thousands of years” before the movies: The Sith were founded, quickly killed each other off, and then started following the Rule of Two and using puppet entities to take over the galaxy.

1000 years before the movies: The Sith ruled the galaxy through a puppet entity, but then the Jedi seemingly wiped them out and overthrew the puppet entity. The Republic was then formed.