It's neat to think about.

Assuming you're just using your main three dots, and assuming you can somehow time them to never clip or fall off, you'll recast bio 3.33x a minute, miasma 2.5x a minute, and bioII 2x a minute. With contagion these decrease to 2.5x, 1.88x, and 1.5x a minute. Assuming (again) your casts are 2.35s, this saves you about 4.6s. So about two casts, which you'll probably use to cast ruin twice at 80 potency, but at the cost of garuda losing a 100 potency wind blade, so your net gain is about 60 potency.

On the other hand, you can't really perfectly line up your casts like that, so the real benefit is higher. Also the above assumes you'll spend the extra time spamming ruin, which you might not. Also you can time contagion to take advantage of buffs.