You can find the corresponding option in the Reagle App: Settings ->Edit button ->Auto-Relock. Two parameters can be set there to configure the re-locking time for your Reagle Smart Lock:
1. The time period (in seconds) after which auto-relock will happen in general.
2. The time period (in seconds) after which auto-relock will happen in case you have activated auto-unlock. This option serves to make sure you don't accidentally unlock your door and then leave it open when walking by outside within a set perimeter. Also, you could choose to extend the relock time here in case the is door auto-unlocked earlier.
Make sure to sync with your lock afterwards to apply the changes.