You can adjust the duration for which the tooltip will show by modifying the data template that will be used to render the data points. For example, for a column series set UseDataTemplate = true, and specify this data template:

        <DataTemplate DataType="{x:Type igCA:ColumnChartTemplate}">
            <Border Background="{Binding Path=Fill}" BorderBrush="{Binding Path=Stroke}" 
		            BorderThickness="{Binding Path=StrokeThickness}"
                    ToolTipService.ShowDuration="120000"
                    ToolTip="{Binding ToolTip}"
                    >
            </Border>
        </DataTemplate>

Let me know how it goes. You can check the documentation for more information about specifying data templates for the various chart series.

-Graham