Probably around 2006. I remember my girlfriend (a casual fan) saying something like (sarcastically) "I wonder who's gonna win?" at a PPV we watched at a bar. So even THEN they were beginning to shove Cena down our throats.
So I'd say it was around the start of "The Marine" Cena. Or maybe even to pinpoint it, when he won the WWE Championship and WWE turned it into that bling-bling spinner atrocity.
Ok I had to add this direct quote from Cena's Wikipedia article due to its sheer hilarity.
The negative reaction towards Cena intensified when facing Rob Van Dam at One Night Stand. Taking place in front of a crowd of mostly "old school" ECW fans at the Hammerstein Ballroom, Cena was met with raucous jeering and chants of "Fuck you, Cena", "You can't wrestle", and "Same old shit". When he began performing different moves into the match, the fans began chanting "You still suck". Cena lost the WWE Championship at One Night Stand, with Van Dam pinning Cena after interference from Edge.