Microsoft not only knows it, they published it also: http://windows.microsoft.com/en-us/windows/lifecycle
That's more than some other software companies do. Let's take Android as an example of that.
Does anyone know how long Microsoft will keep the updates going on 8 and 8.1 operating systems. Will they force us into 10 by not updating? I read that they will ask manufactures to stop making the 7 and 8 systems soon. So where does that leave us on updates?