You're purchasing peace of mind. Its just like insurance in blackjack. It never makes mathematical sense. But if you're entire life savings where on the table on the blackjack table you might would buy the insurance in blackjack, too.
Why? Because peace of mind is damn valuable.
It never mathematically makes sense to have full coverage. The insurance company is very diligent to make sure they're always charging you slightly more than the risk level associated to you. You can think you might understand your driving patterns better than they do, but they understand repair costs, depreciation, etc far better in general because they have huge teams of analysts working on it every day.
So the answer is...you should stop covering full coverage when the value of the "administrative costs" + profit for the insurance company, which is fairly low, is worth more than the peace of mind you get from having full coverage insurance. The more cars you have, the less you need that peace of mind. The lower value your cars are, the less you need peace of mind.
Insurance is very, very rarely a good gamble. I say this and people will jump out and disagree with me, but it really is basic math. So long as the insurance companies are returning profit its fairly evident. Its like vegas, the house always wins. In this case, the more they pay you out, the more they will charge you in the future to make up for it.
The bet may pay off-- go to vegas and you'll see tons of people make money off of bad bets. I waatched a guy split 10s once and get two blackjacks. But in reality without knowing whether it will or will not you're just buying security and peace of mind.
How much is it worth it to you? To most of us, its worth it for quite a while. For some people, its worth it forever. To others, they'd prefer play the odds and end up with a little more money over a long time frame.