State laws differ and some states have no guidelines at all regarding what percentage of a car's worth makes it declared a total loss.
Many states only define a total loss of a motor vehicle as when a car is not repairable, or when it costs more to repair than what it is worth. When a state has not set forth specific guidelines then the insurance company normally has the right to determine if a car is a total loss or not using their internal guidelines.
Without a specific state law as a guideline most insurance companies will determine a vehicle to be a total loss when it is between 51 to 80 percent of its actual worth or basically when the repairs costs are more than the value of the vehicle.
To find out if your state has any guidelines regarding when a car should be determined to be a total loss contact your state's insurance regulator. If they do not have regulations regarding this and it is left up to the individual insurance carrier then ask your insurance agent what your company's guidelines are. If you are in the middle of a third party claim then you can discuss with the insurance adjuster for the insurance company working your claim what their guidelines are for totaling out a vehicle.