Study suggests that whiplash recovery level off 3 months after the accident
A meta level study, Course and prognostic factors of whiplash: a systematic review and meta-analysis, conducted by The George Institute (University of Sydney in Australia) took a look at previous research on accute whiplash injuries. From the abstract:
These data suggest that recovery occurs for a substantial proportion of subjects in the initial 3 months after the accident but after this time recovery rates level off. Pain and disability symptoms also reduce rapidly in the initial months after the accident but show little improvement after 3 months have elapsed.