If Ditto has 60 damage on him and the defending pokemon is a 120 HP pokemon, and Ditto knocks him out, is Ditto knocked out immediately or does it depend on what pokemon the opponent chooses to replace the active with?
In game terms, there is no time when your opponent has no Pokemon active.
So, it depends on how many HP the new Active has.
This is the main reason Fossil Ditto stopped being played when 30 HP Baby Pokemon were in every deck.