Some cases take months. Some cases take years. To a large degree, it depends on how long it takes to find out how badly hurt you are.
If your injuries are very minor or very major, then the case will likely be resolved more quickly.
If they are somewhere in between, it may be months to a year or more just to know how badly hurt you are. And until we know how badly hurt you are, we can't even begin to put a value on your claim.
Some injuries, such as the typical neck and back injuries, which are caused by auto accidents, are notoriously difficult to predict.
An injury to a spinal disc might not be diagnosed for several months after the crash, until conservative treatment has failed to make things better. After a herniated disc is found, usually by an MRI, the doctors still may wish to try conservative treatment for as much as a year before making a final decision that surgery is (or is not) necessary. Until that decision is made, there is no way to value a case.