How much of the rental is the insurance company liable for if their policy holder is at fault?

The insurance company is GENERALLY responsible for 100% of a rental to provide basic transportation or transportation that was of similar size to what was wrecked WHILE YOUR CAR IS BEING REPAIRED! Once the car is repaired, or once your car is written off and a check is given to you so that you can get a replacment car, the insurance company is no longer responsible. These are general guidlines and may vary in your state.