Good question, I've been wondering myself. I have lifted weights since I was 12 and my mom traided weightlifting instruction from the Olympic assistant coach for me for art lessons for his son. Biking is the first thing in 28 years that I've managed to get more addicted to.
I do a 3 or 4 day split for the weights and ride about 5 days/week. I used to plan my leg routines and do them right after Tuesday night time trials, then just not ride again until Friday. Lately, I've been doing my leg routines mostly without weight (lunges, squats...) and that way it doesn't effect my cycling.
I was thinking about taking January off the bike and hammering the leg routines that month. They (who are they anyway?) say you should take some time off the bike every year.
But, I know John Schlitter (a recumbent racer) added more to his leg routines and says it greatly improved his speed and distance riding.