Turning him heel right now would be the best time to do it.
I mean, start planting the seeds for the turn from right now.
They have a great storyline going on right now with him searching for that illustrious 16th title. Have him fail. This pressure takes a toll of him and then Cena snaps.
They also have this Taker feud planned which would work like charm if Cena turns heel about now.
They missed a great chance during his feud with Rock.
I think a better time to turn him heel is at WM-but do so in convincing fashion. In other words, push the PG envelope-beat Taker down to a pulp where he's busted wide open and is on a gurney at WM. Go full scale heel and create the shock factor-much like Taker's loss to Brock at WM30. Shock the hell out of the crowd and make them go "WTF." Then have Cena flip the bird to the entire crowd where he is booed mercifully out of the arena. Then shake hands with the devil himself (Vince/authority, whatever) lol
....then again Cena is too nice and too corny of a guy to do something like this.