If your doing it, say as part of an exercise program, and want to make sure you ride a certain amount, it's easy to track distance over a period then time over a period, to make sure you reach that goal. So the poll is to determine how to best accomplish that. The actual distance could be any number on any scale, I picked km because around here all road signs are in km, so it makes sense to use that scale, 100km per week is a decent amount for someone who is doing so for a little exercise, without requiring a huge time investment, in that it can be accomplished in about 5 hours a week. Of course there is no reason why you could not exceed that number.