1: m_DispatcherTimer = new DispatcherTimer()
  2: {
  3: Interval = TimeSpan.FromSeconds(0.25),
  4: IsEnabled = true
  5: };
  6: 
  7: m_DispatcherTimer.Tick += delegate
  8: {
  9: CommandManager.InvalidateRequerySuggested();
 10: };
 11: 

http://robburke.net/2008/04/23/wpf-command-pattern-when-does-it-query-canexecute/