A snapped neck in cinema generally always results in a virtually instantaneous kill. This is of debatable realism though. Real life is less clean, and whilst paralysis below the neck is likely to happen instantly, death is not (unless combined with a prolonged strangle, or choke, which is what happens in a hanging).

However, this is the MDDB, so lots of entries here are likely to be fatally deft neck snaps at the hands of a Seagal-like figure!

[Thanks go to Matt for providing the icon!]