The general rule is when the car is seven years old.
A greater question is: Can you afford dropping the insurance? Do you have enough money in the bank to allow yourself to move to a lower tier of insurance? If you got into an accident how would that affect you financially?
Your age, sex, marital status, multiple policy, etc will all affect the price too. If you're over 25, married, have kids, also have home owners ins; then the insurance might be worth it to keep full coverage. It may only be $10/month extra for full coverage if you meet all the above criteria.