NVIDIA is projecting 5-10x gains in Deep Learning performance out of their next generation hardware (Pascal) [1]. Might be sooner than you think.
[1] http://techreport.com/news/27978/nvidia-pascal-to-feature-mi...
[1] http://techreport.com/news/27978/nvidia-pascal-to-feature-mi...
Perhaps longer given the restrictions (and huge problems) in further down scaling of the known silicon technology. GPUs are now at 28nm and the newest Intel CPUs on 14nm.