Originally Posted by Kurgan
It's always been an assumption based on an interpretation of Ben Kenobi's "for over a thousand generations, the Jedi Knights were the guardians of peace and justice in the old Republic..." from Star Wars (1977). They interpreted a "generation" as 25 years, and multiplied it times 1,000, hence the number.
I could be wrong, but I think the more literal reference would be to the Ruusan Reformation, 1,000 years before the events of the Death Star and Yavin, where the Jedi and Sith duked it out, and the Sith 'lost,' and the Republic dropped all of their armies and allowed the Jedi to protect the Republic.
Which makes sense, if you look at Kenobi's comment. He says the whole Thousand Years, Jedi Protectors thing, and it coincides with the Ruusan Reformation, which could be a Retcon. The Republic disbanded armies, Jedi became the protectors, that sort of thing.