I have a duck (a bit more than 3 kilos) and two additional breast pieces (each around 300g). Now I am wondering how long this should stay in the oven. Rule of thumb is usually one hour per kg. So I would put the duck in the oven at 180° for three hours. But what about the additional pieces? Should I add them after two and a half hours? Or does this extend the oven time?
Anyone who tries to match the variability of an oven (over time, no two heat identically, even the same brand, and most are inaccurate) with the irregularity of the food (no two whole ducks have the same mass and shape, and the side breasts don't have bones, which affect heat conductance) is doomed to be disappointed. Anyone who suggests that you put in the breasts X minutes before the bird is done is leading you down a primrose path.
By all means, use the rule of thumb to give you a rough guess, but don't plan your meal around it. Get yourself a probe thermometer -- two, in this case: one for your whole bird and one for your separate breast. Pull each piece out when it is done. Plan your menu around that reality.