It'll be interesting to see whether IE11 will go out of support before Windows 8.1 does, which is currently in 2023
http://windows.microsoft.com/en-gb/windows/lifecycle
It's possible that going forward IE11 support will be based on the platform - so IE11 on Windows 7 will stop getting updates in 2020, and then on Win 8.1 in 2023.
If I had to guess I'd expect that IE11 in Windows 10 will be aligned to the Windows 8.1 support life cycle, as it's basically the same version that's in Win 8.1 (minus the full screen / Metro version)