How long any graphics card will stay relevant depends on context, like the resolution, desired framerates and detail levels, game engine requirements, the CPU in the laptop, etc.
Looking at how old a GTX770 is, seeing it continue to handle the vast majority of games played today pretty well, and knowing the rate at which GPU development is slowing, you can kinda extrapolate how long your GTX1070 will remain relevant. That is, unless magically AMD steps up their game and blows the market away.