The military has done a great job of recognizing that soldiers surviving concussive injuries are very prone to long term effects. A TBI (Traumatic Brain Injury) often begins to show up in terms of symptoms within 6 - 8 weeks after an event. Symptoms most often observed early on are insomnia, anxiety, depression, ADD like symptoms (inattention, emotional outbursts like quick anger, boredom).
This is true in the non-military as well. Falls and car accidents can also damage the brain. That proved true for me when after a severe whiplash car accident in college I began to experience symptoms about 6-8 weeks after the accident.
Here is a good article talking about TBI's and how they are more prevalent than we ever thought.