Its very difficult to predict, as Sony and Microsoft will be probably hoping for a 6-7 year cycle with their current consoles, making it 2019-2020 before the new tech arrives. Nintendo however favour a 5-6 year cycle and are 2 years along so they may release new hardware in 2017-2018. This will be very interesting if this happens. Even the big N realizes that the main obstacle that they currently face with 3rd party support is the fact that PS4/PC/Xbone games cant be easily ported over to Wii U and devs don't want to take the risk.
Could it be that Nintendo launch something significantly more powerful than Sony/Microsoft 2 years before the competition are ready to upgrade?
If you really want to know what the next consoles will perform like look to what the very best PC's are doing today and that's about right.
One things for sure, if I have to endure a 3rd generation in a row where 1080p/60fps isn't standard I am going to shit a chicken.