I might get a lot of flak for this, but here goes.

1) Pop your CDs when you expect heavy damage/there's a tankbuster coming up. Keep Delirium up if there isn't a MNK and the tankbuster does magic damage for extra mitigation ofc.

2) If there's a MNK, you can feel free to use Souleater with or without DA every combo. It's a great self-heal, and if you're honestly relying on that 20 extra potency from the Delirium combo, your group has other problems.

3) If I'm pulling a massive group of trash, such as in A3, I pop Blood Price and alternate DA Abyssal Drain with Unleash. If the fight goes on for a whole Blood Price CD, then I guess I get about 5-6 Abyssal Drains in with DA.

4) I add DA to C&S if my MP's doing great and I know I won't need another free Syphon within the next minute. So basically the beginning of the fight is the primary time when I use DA C&S, with a couple others here and there if you're not having MP troubles.

5) Basically what Phoenicia said regarding DP. In single-target situations, never pop DA on it, as the mana could be much better spent on a Souleater combo.