How long does it take for a brain injury symptoms to appear after a concussion?

Seconds to days. Most symptoms develop immediately or within a few minutes. On occasion, concussed patients may not notice symptoms for several days. One risk occurs in sporting activities when concussed patients have minimal immediate symptoms and are allowed to return to a game on the same day. Any athlete suspected to have a concussion should be held out of competition until cleared by a physician.
3-7 days. In general the majority of people resolve these symptoms within 3-7 days. However, it is highly variable and may be longer in children. The important thing is to "rest the brain", and for kids, this includes computers and texting!