There is no cool down for ToC. You can use as many as you want, whenever you want, the effect will just last until you kill an ultra. They do not stack effects.
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But the description says they're stackable tho
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Bungie reduced the effectiveness of 3oC if you use 3oC and then finish a Crucible match or kill an ultra boss in succession too quickly. This was to stop people like me who picked up 25 exotic engrams on the first day 3oC was available. (Best day ever!) There is no single answer on how long you have to wait. Personally I see best results with at least 10 minutes between the end of a crucible match or ultra boss kill.