I actually keep feet/mile statistics. I find it an interesting statistic yet very subjective. I tend to think that if I have a gear that I can comfortably ride most of the hills in, then it wasn't too hilly (hilly, but not too much). If I run out of gears too often, that is a hilly ride.

Around home, that means 50 feet/mile (about the max I can get) is hilly if I refuse to use my low gears. But, if I use the right gear, 50 feet/mile is just a nice ride to me. I've never been on a ride with 75 feet/mile where I've had enough gears---yet--I'm still hoping for the day.