How hard does someone have to hit their head for it to cause a brain bleed?

Variable: It depends on the direction of the force acting on the head and where the head is hit. It also depends if the head strikes anything after the initial impact. A moderate blow to the head can cause bleeding, but this isn't that common. The harder the impact, the more likely that bleeding will occur.

Depends on various: Factors. For example if one is on blood thinners, what part of head makes impact, how weak the bone is. If there is any concern, go get an urgent evaluation. If there is severe headaches, persisting disorientation, nausea, vomiting, confusion, delirium, go right away to get evaluated.