2018 earliest and 2025 at least

NX handheld + console = late 2016
Xbox Two = late 2019
PS5 = late 2019
NX-successor = 2020 or 2021
Vita-successor = probably never... or a VitaHD/Xperia hybrid (Sony handheld that plays all Android games + PSP games + Vita games)

All release dates are set too soon. NX will be 2017 and will offer a drastic change with the gamepad, so Microsoft won't really care about it whether it sells 20 million first year or 20 thousand.

Microsoft and Playstation won't make a move until 2020. Playstation won't need to make a move, and I think Microsoft learned a valuable lesson about jumping the gun with a console, From the RROD with the 360 to terrible PR with Xbone, the are going to obviously be building their next system, but not reveal it until AFTER Playstation revealed theirs. Not only to get a feel of the competition, but figure out how to one up it. Them coming in second place is not going to be that big of deal to them this gen

NX home console: October/November 2016
Next Xbox: November 2018
PS5: September/October 2019 (originally planned for 2020 but pushed forward due to the Next Xbox's release).

NX: Holiday 2016 or 2017

Xbox 2: Holiday 2018 or 2019

PS5: Holiday 2019 or 2020

Hopefully not for another 6-7 years.

I hope the next gen comes in 3 years time, around the end of 2018. This gen started with a small price tag and that limit the time they can stretch the gen. 5 years looks good to me.

I hope Nintendo will release in 2016. I don't think we will see a new Xbox or PS until 2018 and 2019 but I wouldn't rule out that MS will try to release a tablet console earlier.

Nintendo Nx.....2016 or 17
Xbox 2 ...2018.....I believe they feel this generation is lost to Sony...so they will want to make this current generation short and start the next generation early.......
Sony PS5 ...2019 ...I believe if Xbox 2 releases in 2018 Sony is not gonna wait 2 years to release a new console...that is too long of a wait..it would make the PS4 seem like an old system..whereas Nintendo and Xbox have new machines....in reality Sony wants this generation to be long ,but I have a feeling this generation is gonna be shorter than usual because both Nintendo and Xbox want a fresh start...and Microsoft is willing to take a loss this generation(they have the money) to get a head start on Sony next generation

NX next year, no sooner than 2019 for the other two.