How much cardio exercise should I do a day/week to prevent cardiovascular disease?

Depends. Prevention is an important modifier in your question. Prevention must be balanced against innate risks you may have. That being said, movement of almost any type on a daily basis is best, but "cardio" activity of 30-40 minutes 3-4 times per week may suffice to maintain conditioning. That's a big leap, though, to "prevention". Need to be vigilant against overhyping prevention.