It depends what kind of seagull you are asking about. The smaller gulls like black-headed gulls incubate for 21 to 27 days. The larger Herring Gulls are around 26 days and Greater Black Backed Gulls 27-28 days. Both parents share the incubation, so assuming each takes an equal share, the answer to your question is that the average seagull sits on its egg(s) for a total of 10 to 14 days.