Hi! I workout everyday. I do cardio 6 times a week. I lift for 3 times a week. So 3 of those days, i only do cardio. Should I take a protein shake after my cardio (on days i don't do weightlifting)? Thanks a lot!

Yes, you will always want to keep your protein levels on the positive side. Six days of cardio is pretty tough. Make sure you get simply carbs in you after your resistance training as well.

A friend of mine who writes for t-nation calls for caffeine and protein before bouts of cardio and the normal protein carb mix afterwards.

If your stuck for time than yes, a shake with some other source of fuel i.e. fat/carb would suffice however a whole food meal would be better as it takes longer to digest.

Without knowing your daily caloric intake its difficult to say but cardio is ideally a fat burner, so if I was you I would take plenty of mineral water but no protein shake. Is that steady state or HIIT?