Get a Notification if Something is Left On, Open, or Unlocked for X Minutes
This was a very popular use case in Wink, but unfortunately as of this writing the official smartthings features do not support this kind of notification. You can get a notification when something is opened, but not when it has been open for awhile.
Fortunately, because smartthings allows customers to create custom code, there are four or five different ways to do this. Some will require paying a license fee for a third-party app, others are free but have a more complex set up.
See the community FAQ:
FAQ: Get a Notification if Something is Left On, Open, or Unlocked for X Minutes (2020)