Duration for this time-dependent node. Duration is initialized from loaded video length (default is -1). So it's automatically initialized when you call IsTextureVideo, TextureVideo methods.
cycleInterval is just set to duration scaled by 1/Abs(speed), like required by X3D spec.
Duration (and so, also cycleInterval) is not reset when video is freed (like when you set IsTextureLoaded to False, maybe implicitly by calling TCastleSceneCore.FreeResources with frTextureDataInNodes). This way this is available even you freed the texture video data to save memory.