When do I declare victory.
Either when my opponent agrees with my position, I have learned something, or my opponent acknowledges he has learned something.
You see, because I do not so much argue for demolishing an opponent, but to rather correct gaps in their (or my) knowledge, and to inform those listening in that the other guy might be talking BS.
False information is detrimental to real knowledge and truth. False information, when it surfaces, must be corrected. Why? For the good of all of us.
I am never happier, than when someone can show that what I think I knew or suspected, is false. And on the other hand, if I can't show it, I don't really know it. Arguments with other knowledgeable people are a good place to see if I truly know something or not. Arguments with not so knowledgeable people correct misinformation and hopefully, remove ignorance.