For UFT v11.5x/12.0x, it's not possible. See here: https://eyeontesting.com/questions/17408/can-you-set-an-idle-timeout-on-uft-120x-to-release.html
For UFT v12.5x/v14.x and above:
Navigate to the following LicenseSettings.xml file:
Edit the file and change the **LicenseAutoReleaseInterval** element's value from the default of 0 (disabled) to your desired idle timeout in minutes + 1 (e.g., set to 15 for a 16 minute idle timeout).