How long after a motor vehicle accident would you expect symptoms including memory loss, hypersomnia, and mood changes to be attributable to the accident?

I would love to have the answer to this.

I can tell you what the Headache Classification Committee of the International Headache Society says:

Post-traumatic headache (PTHA) is defined as a secondary headache that develops within 7 days after head trauma (or after regaining consciousness following head trauma) [Olesen, PMID 17116090]. Post-traumatic headache is regarded as chronic (CPTHA) when it continues for more than 2 months after the incurrence of the injury, although duration of 6 months has also been suggested [Baandrup & Jensen, PMID 15658950].

Headache attributed to trauma or injury to the head and/or neck requires that headache must be reported to have developed within 7 days following trauma or injury, or within 7 days after regain...