It's hard to get an exact figure, particularly because of unreported accidents, and various sources. But I believe insurance companies have previously stated it's about one in every 250,000 miles. For the sake of giving a wide berth for unreported accidents, and to not give humans the benefit of the doubt, I've been using the rough figure of 150,000 miles between accidents.

I don't have a great source for it though, and if anyone finds a good source, it'd be fantastic.