After head injury how long does it take before you can be sure there is no blood clot in the brain?

Depends: This depends on the mechanism of injury and age of the patient. Majority of head injuries lead to bleeding immediately after impact, due to disruption of blood vessels in the tissues. If an initial ct does not show any bleeding, risks are relatively low to have a delayed bleeding.

Variable: Typically shows up right away but delayed (slow bleeding) can present days or weeks later, especially in older patients.