if done correctly, it doesnt matter when. Heel turns should come naturally, not out of a blue. The best heel turn EVER was the Michaels/barbershop. Everyone remembers the kick, but few remember that there had been problems with th Rockers for months, with Jannety leaving Michaels in the ring to get destroyed and then Shawn doing the same thing. The reason they were on the Barbershop was to "bury" the hatchet after Michaels had refused to tag-in, costing them the belts.
Another natural turn came when Mankind turned after the crowd chanted for Austin while Foley was getting his but kicked in. As he said, he didnt change the fans did. Much the same way with Brett Hart. Thats why he was always a favorite in Canada, because he never changed, the fans did.
With Cena, the build-up is there, which should lead to some kind of PPV action that is perceived as possible heel, followed by the actual turn on RAW. Have him in a tag team or two on one match at Elimination chamber with an injured Ryder, where he lets Ryder get his ass whooped by Kane, the crowd would boo him even worse, even the little kids would boo him for being a coward. Then on the following Raw, JR, or the King have an in-ring interview with Cena to ask him what happened. Thats when he reveals he understands what Kane has been telling him, embraces the hate, and destroys whoever is giving the interview. This would be the final turn, leving him clearly as the heel at WRM, and leading to a great run as a villain with him and Kane destroying everyone, eventually leadin to a returning Ryder to take him out at next years WRM, where he would see the light, ala Hoagn vs the Rock, or for you older fans, when Sgt. Slaughter reembraced his country after defecting to join the Iron Sheik.
Sorry about the length, just my ideas