Greninja got the most screentime after XY&Z started, so pretty much in the last third of the series. Infernape got more development and started to overshadow the other pokemon after the Lake Acuity battle when Chimchar evolved into Monferno. Also the last third of the series. The other pokemon got a couple of episodes (Grotle evolving/Buizel learning Ice Punch/Noibat learning new moves/evolves, Goodra's cameo for the league etc.).
Greninja and Infernape are pretty even when it comes to screentime stealing.
I'd argue it started before Lake Acuity. In all of the Hearthome, Canalave and Snowpoint Gym Battles, Chimchar took out two Pokemon each, including the ace in at least Snowpoint. (I think Hearthome as well, but I can't quite remember) Include the tag battle, an extra battle with Paul and all of that before Lake Acuity...I mean, this is why I can't say Greninja hogged more time than Infernape. And saying 1/3 of the series is still vastly different, since 1/3 in XY is about 50 episodes, while in DP it's a good 20 more.