How hard do you have to hit your head to have a dangerous tbi?

TBI: Usually violent head trauma causes TBI. Milder head trauma may cause some temporary brain cell effect however.

Research on TBI's: Several years ago, head injuries to lab monkeys were graded on force of impact, and severity of brain damage was recorded. This however, does not translate easily to the real world variables that occur with numerous injuries. Your question cannot be quantified.