When asked how much I know a person, there are a couple of things that run around in my head.
I think you never really stop knowing a person. I think you can never really know a person enough. And it's funny and sad that we think we know people so well that we can be comfortable with just being with them, with just being who we really are with them. I've thought that would be okay before, but when you lose that trust with someone once, you can't really get it back without thinking if all those years were just tolerance.
Anyway, I've found that it's very rare to find people whom you can truly talk with. There are people you can have long meaningful conversations with, and there are people who just chat, scraping the top of the iceberg, the superficial fillers, until one gives up and decides to waste time elsewhere. I think it then matters what you mean by knowing a person because you can say that you know a person well by the many meaningless things you've talked about but you can't say you know who the person is without the layers of ever-changing information.
I've heard before that even when you're married, you still learn things about your spouse that never came up when you were still single. Even when you've been friends for years, there are still things that you never really know about the person. So why do we still ask if we know someone enough?
How do you know? How do you really know a person "enough" that you can say you can trust that person with, say, your life?
Matters of consequence.