How long do concussion symptoms last? how will i know it is safe to workout again if some days i feel great while others i have headaches?

If headaches: are related to concussion it's too early. If due to another issue like a concomitant whiplash ( neck ) injury you can ( and probably should ) do some activities that might speed recovery from the neck / upper back muscle spasm. Avoid even mental gymnastics in the early stages of recovery from TBI as it increases the metabolic demands in the recovering brain. Recovery varies -two years or less

Variable: Concussion symptoms can last from hours, to days to a year. You should not do more than mild exercise until yoiur headaches cease.