Log into your IDE and then go to my smartapps. You will not find left it open in the app until you add the template from the IDE. In the IDE select add a new smartapp, the button on the top right side of page. Then select from Template, scroll down until you find the app you want. I don’t think “Notify me” will work. It will notify you when a sensor is open or closed and you can set how often you get a notification, but you want it to check how long a sensor is open. I think " left it open" is your best choice. RemindR may work but I am not familiar with it.