When did Canelo's prime end?

He is still in his prime.

 

Loads up on all of his power shots, which is why he is fading last 3rd of his last two fights.

As someone else mentioned, abandoned his boxing too much now.

If Golovkin had targeted his body (a trait GGG seems complacent not to do at all these days), he would of slowed Canelo down even more.

He is obviously only going to decline from here, I don't think he's massively declining but he will lose the way he currently is fighting, against a patient and good opponent, certainly at LHW.