How long should my chest be extremely sore from seat belt after a car accident?

Car accident: Depends on the extent of the injury & what the x-ray results & medical assessment showed. Review with your doctor.

Ouch: This is variable - could be several days for some or several weeks for others. It depends on the force and your body's muscle mass/tone. The bigger issue is if you have trouble breathing or sharp chest pains, this could indicate internal injury. If so, would talk to your doc about getting a chest x-ray tomorrow am or go to an er. Otherwise, anti inflammatories like Ibuprofen will help muscle pain.