Great question! If the domain was placed in auction by the Registrant (owner) and does not sell, it is up to the owner what they wish to do with the domain. They may continue to list until sold, use it for a website, or they can keep it until they let it expire.
If the auction is for an expired domain and is not sold, it will be returned to the Registry. They will generally hold it for 30 days (time at their discretion), then release it for general registration.
TLH - GoDaddy | Community Moderator Supporting you at x.co/247support