Really whatever you can stick with and enjoy is fine. The specific answer will vary by person and what their goals are.
For myself? 4x a week, 45 min-60 min 6mph jog on elliptical was my best cardio. Combined with weight training. At the time I was into walking half marathons and prob was in the best adult shape of my life!
But as a rebeginner, I'm at 30 min 5x a week basic cardio. No weights yet. I want to work back up to previous levels. This article...
Lose Weight Fast with PCOS
does a better job with links than I ever could.
HTH!
A.