I consider heavy less then 6 reps, medium is 6-10, and light is more then 10 reps.
I usually train in the medium range, but I also do plenty of heavy lifting as well, especially on the big compound movements like squat, deads, rows and presses. I'd say my routine is 70% medium and 30% heavy. Calves and abs are the only muscles I train with light weight for high reps.