Ultimately, it will depend on what your goals are and how your body responds to training.
I run approx 12-13 miles a day, one shot at a time, all of which is hill running. Easy days are never in need, but once in a while I run flat for an easy. Since I don't do high end quality stuff like repeats on the track, I more or less just elevate the pace or intensity of the run the last 1/2 to 1/3 of run, always depending on how by body is responding--this is the ultimate factor in everyone of my runs. This has allowed for 80mpw and a competitive level of fitness.
My fitness level right now allows for 70-90min runs without feeling burnt at the end. Fatigue, definately, but not burnt. This type of running allows for consistent training, week-in and week-out (given the terrain you run on and your daily schedule). I work 10-6, m-f, in so cal, so the weather is kind most of the time, but I should wear an air filter on my face!