public class RadScheduleViewExtensions
{
    public static int GetVisibleDays(DependencyObject obj)
    {
        return (int)obj.GetValue(VisibleDaysProperty);
    }

    public static void SetVisibleDays(DependencyObject obj, int value)
    {
        obj.SetValue(VisibleDaysProperty, value);
    }

    public static readonly DependencyProperty VisibleDaysProperty =
        DependencyProperty.RegisterAttached("VisibleDays", typeof(int), typeof(RadScheduleViewExtensions), new PropertyMetadata(0, OnVisibleDaysChanged));

    private static void OnVisibleDaysChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        RadScheduleView scheduleView = (RadScheduleView)d;

        if (scheduleView.IsLoaded)
        {
            scheduleView.Measure(Size.Empty);
                
            Dispatcher.CurrentDispatcher.BeginInvoke(new Action(() =>
            {
                Size renderSize = scheduleView.RenderSize;
                scheduleView.Measure(renderSize);
            }), (DispatcherPriority)3);
        }
    }
}

<telerik:RadScheduleView x:Name="scheduleView"
                         AppointmentsSource="{Binding Appointments}"
                         local:RadScheduleViewExtensions.VisibleDays="{Binding MyPropertyForVisibleDays}"
                         IsGroupHeadersVirtualizationEnabled="True">
    <telerik:RadScheduleView.ViewDefinitions>
        <telerik:DayViewDefinition VisibleDays="{Binding MyPropertyForVisibleDays, Mode=TwoWay}" />
    </telerik:RadScheduleView.ViewDefinitions>
</telerik:RadScheduleView>