Turn 1: Use it everytime it's up
Turn 2: on the random ADS, but not for the boss.
Turn 3: Use it.. whatever...
Turn 4: At start when AoE so it's up on the mid-boss at midfight after the soldiers. Then whenever it's up except at the end on last mob to avoid AoE.
Turn 5: on Prison and Twintania. Try to have it up for the giant snake boss.

Garuda: Use it often
Titan: Depending on your latency, mostly on adds too (Extreme) (i'd rather not use it at all on Titan).
Ifrit: On Pillars

One things to know also, is when to remove the effect.