Probably the best time to make move, 2017. People bought a XBO/PS4 in 2013-2014, you hear about the Neo or PS4k at E3 or wherever, you're not really feeling you've got the most out of your purchase 2-3 years in or not really seeing the benefits of Sony's 4tflop machine and MS say hold your horses we've got something better coming.
I'd imagine by 2017 (4 years in and 2018 (5 years in) and faced with a new system to buy you might plump for the Xbox 6tflop whatever it's called. I know some will say you can just wait for a PS5 then but the new Xbox will be at that ~5 year time, PS4 Neo might get caught in the middle here and be too incremental to be bothering with. Of course Sony has all the WW base, either or both could fail but I think MS really are just looking to get back in the game and get back to decent place at the least. Those 360 owners might jump back.
MS might need to out flank the Neo stuff. Not making a prediction though, PS4 neo could turn into a cheap but decent option and the power game reverts back to PS2 inferior spec days.