Does a chronic hematoma start bleeding as soon as the impact happens? or does the bleeding start days/weeks after the impact?

Hematoma formation takes a little while for the blood collection to develop. It will develop quicker in some areas than others. For example a subdural hematoma in the closed space of the brain may slowly develop over days while a lump from a black eye develops almost instantly.

A hematoma is an accumulation of blood trapped within surrounding tissue. They occur acutely, usually from trauma, and develop within minutes to hours. The exception would be a chronic, subdural hematoma, where bleeding occurs along the brain's surface from small capillaries after a blow to the head, and continues very slowly for weeks to months, often not noticed for a long time.