So, are you referring to nosy questions? Or comments that are just very inconsiderate?
For the former, you can either tell the person calmly that you consider this a personal issue. Or display a confused look and ask them, “I don’t know how this would affect you. Why do you feel the need to know?” Or just stare, that’s always awkward, lol. For the latter, explain why you think their comments are offensive…(it can be hard not to lash back).
As for “how much” you let people make comments…Well, you can’t stop them from speaking. Tolerance can depend upon frequency (after a point, you may think enough is enough)…and sometimes it depends on the degree of idiocy even if it’s a first time offense.