0. We have long passed the diminishing returns. Graphic leaps are just getting smaller every generation. That is why generations are so much longer now. Before a new console could come out 5-6 years. Now it is closer to 7-8 years. I mean if you really want o mess with someone's head.
The Gap Between the N64 and the Wii is only 10 years. Yet the gap between Xbox 360 and Xbox One is 8 years.
Also, higher graphics are also taking so much manpower to make that big AAA games are dealing in numbers and will continue to do so as more companies are either priced out of the market or because they have to release less.