Assuming they target something like a 5x improvement on perf/watt, no less than one year after the 10nm manufacturing process becomes mature for high-performance parts. That is about 2019, I guess. Unless 10nm is a short stop-gap node for both TSMC and Samsung/Global Foundries like 20nm was and never fully matures. In that case, consoles would be stuck waiting either for reliable 7nm parts or be forced to release on 14/16nm. But I'm certaing it will be at least two nodes down. The X360 released two nodes down from where the 6th generation consoles released (with a huge increase on power consumption) to great success, so that's probably enough. Also, this is hardly mid-point. Right now is the same as January 2008 or 2009 for the last generation, depending on how you do decide to count.
