Essentially, the question is, how many GCDs does it take before setting up 3 dots is superior to just spamming ruin?
The answer is...~5 GCDs.
Math: it takes 3 GCDs to set up 3 dots. 1 dot applied at 2.5 sec, 1 dot applied at 5 sec, and Bio being applied at roughly 5.5-6 sec depending on animation delay. During that time, only Bio II gets to tick once. At the end of 3 GCDs, you will have roughly "half" a tick of time with 3 dots, so this is averaged out to (35+40+20)/2 potency.
Opportunity cost is 3 ruins, which is upfront 240 potency. We then subtract the simulated dot damage from the first 3 GCDs, then divide by the total dot potency to see how long these dots will take to equal the deficit. Turns out to be about 1.65 ticks or 5 seconds, so 2 GCDs after the 3 GCD setup.
If you can set up all 3 dots and fester, you will obviously win out...but at the cost of severe mana consumption as well as loss of potential dps (because you could've festered the boss instead, without losing the setup dps).
Another solution is to attempt different combinations of DoTs. Obviously since Miasma's contribution sucks, we will drop that first.
Bio II > Bio will overtake Ruin spam at roughly 9.4 seconds or 4 GCDs.
Pure Bio will overtake Ruin spam at roughly 6.5 seconds, or 3 GCDs.
So it takes planning, but you can find the optimal target swap rotation on small targets.
<2 GCDs, spam Ruin
3-4 GCDs, Bio > Ruin
4-5 GCDs, Bio II > Bio > Ruin
>5 GCDs, full setup