Duckling should be cooked a long time in order to melt the thick layer of fat that is beneath the skin away completely, leaving the skin very, very crisp with no fat at all left between it and the meat. Rub the duck well inside and out with salt and pepper, adding, if you wish, garlic and lemon juice. Cook it on a rack in an oven preheated to 350 degrees F for 2-1/2 hours, depending on size. A stuffed duck will take longer than an unstuffed one. Prick the skin before roasting the duck to allow the fat to drain. When the duck is nearly done, take it from the oven and remove all the fat you can from the pan. Baste with its juices and any marinade that you may wish to use. Duck is delicious but wasteful. A 3 to 3-1/2 pound duck serves two generously. A larger duck can be carved to serve four.