Mix the concentrate and milk together. Pour into a shallow dish. A pie pan works very well. Freeze until solid, approximately 3 hours. Remove from freezer and let stand at room temperature until you can break it apart, approximately 20 minutes. Put the broken pieces into a deep mixing bowl. Beat with an electric mixer until it forms a smooth slush. Pour into an airtight container and return to the freezer. May be stored up to 2 months.