Medical background here. One to two weeks sounds about right for a fading bruise as you describe. A hard kick or punch could leave a bad bruise but not injure abdominal organs if it was centrally in the belly, closer to the navel. Up just under the ribs you'd risk damage to the liver or spleen, and too laterally at the sides and you might bruise a kidney, although that's more likely with a blow from the side or back.