I am so sorry to hear that you are experiencing issues with your Note8 display. Screen burn does occur over time and depending on your usage with the device, it will determine how fast Screen Burn occurs. It is highly advisable to NEVER keep the display brightness turned all the way to the max for long periods of time. Maintaining the "adaptive brightness" feature will help medigate the screen burn. Other methods for screen burn control are: Utilize "Dark Mode", or Apply "Blue light filter".
If you were to start applying these recommendations to your device it would definitly slow the progression of the screen burn. Furthermore I would highly recommend that you call 1800SAMSUNG to have them verify your IMEI to confirm warranty status.
I hope this helps,