To clarify, this is not a feature built into Ignition. The screen lock is a script located in the templated projects you can choose from in the designer. This script checks the inactivity of the client every 5 seconds. After a 5 minutes of inactivity the client will lock. If you create a blank project the script will not be present.
Template projects are selected here
The script can be disabled by following these steps:
1. Launch the project you would like to disable the auto lock functionality
2. Go to Project in the menu bar and select "Client Event Scripts"
3. Select "Timer" and you will see a Client Timer Script called "Check Inactivity @5,000ms"
4. If you no longer want the script you can simply delete it.
Or, if you want to disable the script you can double click the timer script name and set the Enabled property to false.
5. Finally, save and publish your project.