My DD12 has had four concussions.
As her sports medicine doc says, concussions are like snowflakes. They are all different.
Some of her symptoms include: sensitivity to light/sound, headaches, mood changes, personality changes, tiredness (falling asleep very early in the day), loss of appetite, balance problems, inability to concentrate, headache when reading/watching TV/any cognitive work.
Protocol is pretty standard. Avoid anything that brings on symptoms. Gradually reintroduce activity (TV, school work, etc) as tolerated without symptoms. She has also done physical therapy to restore her balance.
She has been cleared as early as two weeks after diagnosis and as late as three months. When the patient is completely symptom free s/he is cleared.