Scruffy said:
Within a few days of TPM's premiere. That's when I knew that TPM sucked, anyway. Took a few days for the OMGSTARWARS glow to wear off. I don't know when I decided that the prequels, as a whole, officially sucked. Logically, I could not have made that decision until I had seen all of the prequels, which didn't happen until some months after RotS came out. (I had to see it a second time to catch the bits I slept through the first time.)

Ditto.

It began when I saw TPM and realized that there was just something missing that wasn't in the classic trilogy.

Then the fan-boys started in with their near-violent attacks on any critic that had anything even slightly negative about the precious franchise. That didn't win them any votes from me.

It became Officially Official when I realized that I didn't have to like the prequels just because I considered myself a Star Wars fan. Parts of them may have been enjoyable; but just because I realize they are movies that just don't work, doesn't make me a hater.