I'm not sure if you have found the answer but I also found out that it got reset to the default of 4 seconds when we upgraded to 8.8 SP2.
In order to change the PXE Boot Option timeout, you must login to the LDMS Console on the Core. Then go to Configure > Services. On the OS Deployment tab is where you can set the desired seconds to display (up to a max of 60 seconds). Go back to the PXE Boot Menu and click Update and then re-deploy the PXE representatives.