You need Graded crystals (Usually D Grade) to use cubics. You summon the cubics by having the crystals in your inventroy and clicking their skill icon. Once summoned they hover over your shoulder until they 'trigger'.
From all tests, their trigger rate is tied to your attack rate. If you want to trigger them more the only thing you can do is increase your attack speed. (or use a weapon that attacks faster.)
Matk has no effect on cubics.
Cubics last about 10 Min. (it's a reall odd number and doesn't match your buff timers very well at all.)
They last 15 minutes. The cost of using cubics for day to day grinding eats into profits, so its best to just leave them for raids, hardcore dungeons and PvP.
For the debuff and posion cebic maybe, but the Vamp cubic rules. But they all really help, SK gets really **** cubics.
Vamp cubic can spellcrit, so thats cool. I usually have at least vamp cubic out, and I'm hard pressed for cash, i break even usually or make a tiny profit soloing on my SK with my vamp out.
Cubics are pretty helpful at times...only way to gauge the time for recast is by getting buffs or def. or atk. aura and waiting for the 4 minutes left mark. But save em for PvP