That's like asking someone, "What's the maximum number of times you will pee in a year?" The answer of course, is, however many it takes.
I take the odometer reading on January 1, and subtract that from the odometer reading on December 31. That's the total miles I've driven for the year. From that number I subtract the number of paid miles I've driven for the year, and the result is my deadhead miles for the year. I divide the deadhead miles by the total miles driven, and it better be 35% or less, otherwise I've made some bad decisions in the previous year that I need not repeat in the coming year (like chasing freight when I should have sat, going home too often or running around too much while I'm at home, etc.). Now, if that 35% is 375,331 miles, so be it, because that means I drove more than a million miles last year, with more than 697,000 of them being paid miles. I'm tired just thinking about it. But that's OK, 'cause now I'm rich. I need to pee.