There're all sorts of sources telling you to lift weights three times a week, minimum twice a week. But life gets in the way and messes up your schedule. Plus if you go cycling on your off days, for people who like to go long distances, you might be too tired to weight train the next day.
And then there's the study that says if you do cardio more than 45 minutes, you'll be eating into your muscle mass. This explains why marathon runners are so skinny.