How hard does someone have to get hit to rupture their spleen?

Depends on quality: of the spleen. If its a healthy, then considerable amount of force would be needed, or a rib fractured in that area can puncture it. A fall onto left side causes the left elbow hitting the splenic region is a known way of splenic rupture. If the spleen is diseased (weakened) from any cause, then a minor trauma can certainly rupture it.