I like to distinguish between whether I enjoy them, vs. whether they suck. My conclusion over time has become that TPM sucks, but I honestly enjoy it for unknown reasons. AOTC sucks and I pretty much only enjoy laughing at the parts that suck the worst. ROTS just sucks period and I can't make it from one end of the movie to the other without constructing a noose and hanging myself.

As to when I realized it - I guess it was in the theater during AOTC when, during Anakin's sleeping/dream scene, the audience started laughing.