i don't believe you can set a delay for nnm registering an ov_node_down event... ov_node_down is not configurable, like the threshold events (which you can configure only to alert after the condition is registered for a specified period of time). you could configure your additional actions to wait a certain period of time before launching... but this would serve little purpose because ov_node_down is still registered in the event viewer immediately, and the additional actions would merely pause a bit before launching (they would not be canceled if the node went up before they launch).
hope this helps!