What is your goal for cardio?

Why are you doing it?

If you are doing it to burn fat, that's one thing. If you are doing it to work for your health, that's another. if you are doing cardio just for the benefits gained in weight training, I would say you are doing way too much. Of course your intensity plays a large roll as well.

DC trainees typically do cardio as well, but it is only on the off days and it is done with relatively low intensity.

If you are doing cardio for other reasons, then you just have to decide which is more important to you. I personally belive that if you are using any intensity at all, you are doing so much cardio that it might very well be eating into your mass gains.

I lift legs twice a week and do conditioning work and they get bigger stronger

depending on how heavy you go, training legs more than twice a week along with cardio may be risking overtraining and will limit any growth.

Cardio is a good thing.

If you like to lift and run, then giv'er. If you workout for the fun of it and don't plan on doing contests, then who cares if running will slow down your leg growth.

So you end up with 28" quads and not 30" quads. But you will have a strong heart and all the other benefits of cardio work.

I used to be all about getting huge and would never do cardio. I was afraid that cardio will slow down my gains. Well, I am never going to be a pro BBer, and probably never enter a contest (but who knows). I would get tired walking up a flight of staris... for what. So I can be 240lbs and out of shape.

Keep lifting
Keep Running

If your knees start to take a beating, then look towards some other forms of cardio.

You can also get cardio benefits with high repetition squats