When someone is knocked unconscious, how long of a delay can there be between the impact and the loss of consciousness?

Usually immediate. Most times loss of consciousness is immediate. A delay of minutes to hours would be a cause for alarm and an immediate visit to the emergency room as an internal bleed in the head would need to be ruled out. Usually someone is knocked out (right away) for seconds to minutes and that in itself warrants and er visit. Any loss of consciousness basically needs to be evaluated in the emergency room.