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Unfortunately, the InkCanvas has no useful events for you to use in your scenario, but the InkPresenter does. So in order to implement your requirement, please register those two stroke events: StrokesCollected and StrokeStarted, then you could start and stop a DispatcherTimer to figure out whether your user has stopped drawing or not.
For more information and samples, please check the 'Dynamic recognition' section on this official document: Recognize Windows Ink strokes as text and shapes.
Thanks.