I do both.

During the week, I go by time. When I can ride at lunch, I go as far as I can in the time I have. These miles increase as the season goes on (the routes are pretty much the same).

After work, I go as far as I can in the time I have. Sometimes I'm limited by daylight, sometimes by other commitments.

On the weekends, I go by milage taking into consideration conditions, terrain and my body. Training for mountain centuries, I basically alternate weekends of long (65 - 85 miles) moderately hilly mile routes with shorter (40 - 60 miles), mountainous routes.

I also commute a couple of days a week and I'm trying to adjust my work schedule to allow me to do more.