Chosen One: Force Out of Balance
It is now that the force is widely out of balance in every form. The Jedi grow and number in the thousands. Their ways influence the galaxy, largely unchecked and they even become an unofficial/official part of the government. The Jedi ways are the only ways for a thousand years.
Meanwhile, the Sith number merely two. Although through planning and plotting against Sith masters, there were many times multiple dark side users vying for Sith power, but only two carried the title. Both in number and power, the Sith were minuscule.
Unfortunately, we do not know the exact time when the prophecy was written. One would have to think it was in this era somewhere that the prophecy was given to the prophets by the midichlorians. (You can hate ’em but they are canon).
Time of the Prophesy: Chosen One and Beyond
Oddly enough, in Claudia Gray’s latest book, Master & Apprentice, we learn a great deal about prophets, prophesies and the Jedi attitude towards them. There used to be a sect of Jedi that focused on studying prophecies and tried to intuit their meanings.
The Jedi order, however, began to distrust this use of the force as it often led to the dark side. Learning turned into manipulation or learning to gain advantage over the future. The practice was more or less stopped with occasional Jedi dabbling in it, but then moving on.
Jump forward 1000 years and one of those Jedi went by the name Count Dooku. Dooku was obsessed with prophecies for some time, but eventually made the show of putting them away after discussing them with his padawan Qui-Gonn Jinn. In reality though, the prophecies did consume Dooku and we all know where he ended up.