[Browsable(false)]
    [EditorBrowsable(EditorBrowsableState.Never)]
    [Obsolete("Property '" + nameof(Duration) + "' should be used instead.")]        
    public long DurationTicks { get; set; }

    [NotMapped]
    public TimeSpan Duration
    {
#pragma warning disable 618
      get { return new TimeSpan(DurationTicks); }
      set { DurationTicks = value.Ticks; }
#pragma warning restore 618
    }

Update

This is now achievable since EF Core 2.1, using Value Conversion.

builder.Entity<Stage>()
    .Property(s => s.Span)
    .HasConversion(new TimeSpanToTicksConverter()); // or TimeSpanToStringConverter