The word "know" in the OP's question is a bit vague. I mean, I "know" a guy whose net worth is in the hundreds of millions; have played golf with him a few times, and see him every once in a while. I could claim I "know" Michael Eisner since I used to work in Imagineering and have been in multiple meetings with him.
For someone I know really, really well though the answer would be tens of millions. I know them well enough that I have a key to their beach house on the Gulf of Mexico and use it whenever I feel like making the drive. They live in London so it rarely gets used by them. Really nice place. It even has an elevator. In fact, I'm at the house right now for the holiday weekend.
View from the sitting room window: