This is a great question. A decade ago I would have been a 10 across the board! lol....but this really came from the delusion that everyone was operating at the same intellectual level I was. I've met many people that I completely disagree with, but they clearly and logically lay out their argument and they understand the point I'm making, we just disagree. So I was a 10 because I had difficultly accepting that some people are basically too dumb to have an discussion/argument/ disagreement/ debate etc.
As I've gotten older, whether I know the person well or its a stranger, their level of ability to have, express and receive information to really be worth my effort nearly always determines if I will keep going or just let it go. And George Carlin was right that it takes about 8 seconds to realize someone is stupid

If I see someone as smart enough to understand the issue (the more complex the issue, the more I expect from them) well, then I'll keep at it, because I know they can understand....however if its pretty evident that they are lacking the knowledge, education or mentally capacity to really engage in what it is I just let it go. I wouldn't argue with a child or mentally ill person, because they don't have the same reasoning ability, so why would I argue with an "normal" adult that is incapable of the same. Sometimes people are not "stupid" but they have some other emotional or mental issue that makes them incapable of seeing other points of view like true narcissism or other personality disorders. In that case, I usually don't argue the actual topic (why bother, the person will never admit they are wrong, or even attempt to see your point of view), but in these cases I will just let them know that their personality issue makes them unable to effectively "hear" other people's points so I'm not going to waste my breath.
Again, this is not about agreeing. I've known many people and those I'm friends with that I disagree with on various topics, its about the person's ability to actually discuss these topics that makes it worth my time.