Protecting your NCD usually adds about 10% to the cost of your insurance. If you have an accident you normally only go back 2 stages on your NCD ie, from 60% to 40%, not back to 0%. It works out that 4 years is about the break-even point; in other words if you don't pay to protect your NCD after 4 years you've saved enough money to cover any increase in your premium due to losing part of your NCD. This ignores changes in premium after an accident, which are nothing to do with the NCD.