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.)

BTW, thanks for electing me to the office of he-who-decides-the-prequels-suck.