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

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.
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.

Related Questions

Friend hit me in the head. Not hard, more like a joke. Could that cause a concussion or brain bleed?

No. While in medicine, anything is possible and of course it is possible, it seems exceedingly unlikely to the point of impossible and certainly improbable. Do you have any symptoms? Loss of consciousness? Headache? Difficulty thinking or concentrating? Nausea? Vomiting? If you have none of these sympoms, the answer remains no. If you do have them within 6 hours, perhaps get checked out. Read more...
Not likely. 27 year olds rarely suffer concussions or bleeding from slight trauma. If you are not having any symptoms like headache, dizziness, trouble walking, or nausea, no need to worry. Read more...
Unlikely. It does take certain momentum and force to cause concussion. Brain bleed take even more trauma than trivial hits. Loss of consciousness often is associated with significant concussion. Read more...