How long does it take for internal bleeding symptoms to show up after relatively minor blunt force trauma tothe abdomen below the ribs?

Depends. Trauma to abdomen under ribs may cause internal abdominal injury to different organs including spleen, liver and other organs.Symptoms will show up after different time intervals , instantaneous in case of some organs and delayed in case of trauma to other organs.
Depends . On the amount of bleeding.
Large amounts almost immediately
Otherwise, it can take minutes, hours, or days depending on the causes of the bleeding .
Hours to a week. Bleeding into the abdomen is usually detected within a few hours, dropping blood pressure, abdominal pain, falling blood count, rapid heart beat, sometimes shortness of breath. CT scan of the abdomen or ultrasound of the abdomen will make the diagnosis if there is significant bleeding. If the injury was on the left under the ribs, the spleen may be the problem, can be asymptomatic up to a week.