A recent study showed that neck stiffness--the main symptom of whiplash injury--goes away within a few months. Three months after a collision, people with minor whiplash had the same amount of neck movement as people who didn't have whiplash to begin with. Other symptoms such as neck pain and headaches went away as neck movement returned.
However, these results were based on "minor" collisions. Your symptoms may be different, depending on your accident. A physical therapist may be able to suggest helpful strategies to speed your recovery.