Dear, I looked through many websites and materials for your question. It is really not easy to answer. First of all, a concussion probably will cause many symptoms, such as severe headache, sleeping less, seizures and repeating vomiting and etc. Yet no materials said concussion will cause bloodshot in eyes, unless you hurt your eyes in some way when concussion happens. As to the time when the bloodshot could goes off, it is depending on the cause. Different cases, different rate. There are some common reasons to get bloodshot in eye, smoking into eyes, eyes fatigue, dryness of eyes and etc. For these common reasons, the bloodshot will goes off soon. Yet for viral infection, the bloodshot will clear out in 3-5 days. And for eyes with subconjunctival hemorrhage, it would take about two weeks to clear out. Because of the complicated situation, I can't tell you when your eyes could be clear. If the situation is serious, you'd better see a doctor for help. Hope your eyes could heal soon.
