This is another question with no easy answer. It depends on who is involved. In truck accidents or medical malpractice cases, more than one party may bear liability for your injury. When more than one party is involved, the claims grow more complicated.
An insurance company is likely to make an initial settlement offer after you file a claim. Often, the amount of the settlement is well below what is acceptable. The negotiation portion of your claim begins once the insurer has made the initial offer. In some cases, insurers will negotiate in good faith, while in others they will minimize your claim.
If an insurer is unwilling to negotiate, then the matter may have to go to trial. Cases that go to court usually take longer.