I think it’s different strokes for different folks. Maybe a radio button when you set up the notification that has 3 options:
- Notify me once and only once
- Notify me hourly until traffic subsides
- Notify me daily until traffic subsides
You will probably want to put a little smarts in the notifier so that it buffers up N consecutive readings over the threshold before it triggers a notification. Otherwise if you have it set to 15 visitors and you are at 14, then 16, then 14, then 16, etc you don’t annoy people.
The challenge with notifications is always reducing the noise so that they’re actually useful. Too many and people end up disabling them altogether.