Using a knife, trim skin on drumsticks about 1" below the tip of the bone and scrape down toward the meat, exposing the bone; place legs on a baking sheet and set aside. Grind thyme and bay leaves in a spice grinder; transfer to a bowl. Stir in salt, sugar, pepper, garlic powder, and ginger; rub mixture all over duck, pressing it into the skin and coating completely. Cover with plastic wrap; chill overnight or up to 2 days.
Next day, rinse duck and pat completely dry with paper towels. Melt duck fat in an 8-qt. saucepan until a deep-fry thermometer reads 200°. Add duck legs; cook until tender, about 2 hours. Let cool to room temperature, then cover and chill. The confit can be used right away or will keep up to a month.
To serve: Heat oven to 400°. Remove duck from fat; place skin side up on a baking sheet with a wire rack. Bake until skin is browned and crisp, about 40 minutes.