Combine salt, black pepper, chopped thyme, bay leaves, chopped rosemary, and 4 cloves of garlic in a medium bowl. Stir to mix well. Add the duck legs 1 at a time, firmly packing the salt mixture onto both sides. Place the duck legs on a rack set over a baking sheet, skin side-down, and top with the remaining salt mixture.
Refrigerate uncovered at least 24 hours and up to 48 hours.
Preheat the oven to 225 degrees F.
Rinse the duck under cold running water to remove the salt crust and pat dry. Place in a medium roasting pan without overlapping and cover with the duck fat. Place the remaining 2 cloves of garlic, the sprig of rosemary, and sprig of thyme on top. Cover tightly with aluminum foil and roast until falling from the bone and the fat is completely rendered, 6 to 7 hours. Remove from the oven and let cool at room temperature.
Transfer the duck to an airtight container with a lid, cover with the fat, cover, and refrigerate for up to 2 weeks.