If the Bluetooth connection is lost between the phone and the watch it will try to find it again. The progress is the same as when the user turns on the Bluetooth. The watch will first, broadcast 30 seconds heavily, and after that, less frequently till it has broadcasted for an hour. The duration of broadcasting can be set by the user in the app. In the app, we will give the user a fixed selection where the user could choose from <iOS: 2 minutes till 1 hour> <Android: 5 minutes till 55 minutes>.
So, for example, if you have set the "Scan Duration" as 30 minutes, after losing BT connection, the watch will broadcast for 30 minutes to try to reconnect. First 30 seconds heavily, less frequently after that, until 30 minutes later, if it still can't reconnect, watch BT will be automatically off. It won't try connect again, unless you press any button to turn BT on, it will start the whole process again.
So Scan Duration is actually set as how long you want the watch to search for the App after you've been away from your phone. If you will be back in a few minutes, you can set it shorter. If you know you will be away for quite a while, you can set it longer, like 50 minutes; or on the contrary, you want to save the power and reconnect by pressing the button when come back, you can set it shorter too.