I've tried to make a very rough inference based on John Carmack's statement along the lines "at one order of magnitude improvement on today's GPU's we'll start seeing it in real things, and at two orders of magnitude it'll get competitive in games" (http://youtu.be/P6UKhR0T6cs?t=1h4m30s).
Nvidia's Optix is built on top of their general purpose library, CUDA and it massively accelerates ray casts. People have been using this for gpu accelerates path tracers.
Who predicts this? Path tracing is fundamentally different from rasterization, and I doubt that GPU manufacturers can transition that fast.