Question about yugioh Final Countdown and Pro Clock Of Destiny?

Ok how many turns that is added if there was two Pyro Clocks Of Destiny with that Final Countdown? In like 20 turns will that be cut down into like 10 turns or what? Also does it work after it goes to the graveyard, and just wondering does it work only with cards that has turns in it's text? If it my oppent's next turn does that count as well?