I'd say the length of probably 75% of my rides is determined by time. When I'm out for fun, I give myself X amount of time to get home before I feel like I'm being selfish by abandoning my wife with our baby when she might have something she wants to do with her weekend time too. Or until I think she's going to wake up from her nap. On the way too/from work, if I feel like I've got a little extra time I might add some distance. Speed and distance are fun to look at and I use them with my time to know how much further I can go in the time I have given myself, but really time's the important factor. I could just wear a watch I suppose, but I don't like wearing one when I ride.