How fast will additives finish off an engine?

Nobody can tell you that. Because it depends, first of all, on the final chemical make-up of the "engine oil + additive" mixture, its parameters, and how far those parameters deviate from what's actually allowed for your particular engine.

In any case, if we're talking about a foreign car, we're talking about hundreds, at most thousands, of kilometres. What happens after that? In all likelihood, things get worse than they were before the additive was used, and while earlier you could still consider replacing individual internal engine parts, after the additive's destructive effect you're most likely looking at replacing the whole engine. It certainly won't get better — that much is absolutely certain either way.

The harm an additive does to an engine depends on its design, the materials used, and the chemical composition of the additive itself. If such an engine ever needs an overhaul, its internal parts that can't be replaced will, at minimum, need to be cleaned of the additive — which is far from simple, and sometimes impossible.

Take a look at the picture on the right — that's the result of using an additive. Which additive, you ask? Well, pretty much any of them! That's right, any additive can bring your engine to exactly this state, and the odds of that outcome are extremely high. How much longer the engine will keep running once it's in that state is entirely up to you to decide.