Here is AMD Warranty Terms and Conditions for your RX6800XT GPU Card: https://www.amd.com/system/files/documents/radeon-rx-6800-warranty-card.pdf
NOTE: This Warranty is only if you purchased the GPU Card directly from AMD Store.
Seems like you pay for shipping the GPU to AMD Warranty repair center.
As to how long it will be to get your GPU repaired or replaced it depends on your situation, location, shipping destination, if they have a GPU in stock to replace it with, how long it takes to repair, etc.
Possibly some other Users who have turned in their GPU cards under AMD Warranty can give you an estimate of how long it will take for you to get a GPU card back.
AMD Warranty will notify you if your GPU card is defective or not and what actions they will take.
But the best thing you can do is open a AMD Warranty Request and see if they believe you need to RMA your GPU card to be checked, repaired or replaced: https://www.amd.com/en/support/kb/warranty-information/rma-form
That would be the first step. Then you can decide if you want to go through with the RMA or not.
EDIT: That AMD Warranty Terms & Conditions are also in several Foreign Languages in the same PDF that I linked.