Looking at some of the numbers that schools provide on their websites, and assuming that schools accept anywhere from 1.5 - 3 times their class size, and considering the huge variation in interview philosophies from school to school, I think the number can be between 40%-70%.
In other words some schools invite almost 2.5 times the number of people they can accept to interview, while other schools are very selective about who gets interviewed.
I personally would prefer that schools stick to the higher end of that range to minimize our expenses, however I can think of many arguments that people might have for a number at the lower end.