When does curiosity become nosyness?

I think that your own sharing helps to draw the line. Received wisdom is that people who talk a lot don't listen -my experience is that talking about yourself or general topics encourages conversation.
Broadly speaking, I think similar levels of disclosure makes it even out between people at all levels of friendship. Occasionally someone tells you a lot about themselves - usually during an illness or stressful time - where reciprocating isn't appropriate. And of course, some people drone on about all aspects of their lives and you wouldn't want to share that detail!

But if you're not sharing the same level of information about yourself, or you're having to ask specifically instead of them sharing, it's too much!

It is probably how you ask people. You have to show interest in what other people are doing etc. Don't ever ask what a/b/c costs unless you know each other well and the conversation leads there.

If you want to know how much, whatever it was cost, Google it in the comfort of your own home, mutter how extravagant they have been and then do not gossip about it.

My sister always asks how much things cost and it drives me mad. I usually say that I can't remember because I really don't think it's anyone's business what I choose to spend my money on .

I hate being quizzed to be honest, it ,makes me clam up. I am not that interested in other People's business either. I don't mean I wouldn't be interested in hearing about someone's holiday if they wanted to talk about it, or how their Family are, and yes IF I ask, then I am interested in the reply.

I don't do Gossip either, I am simply uninterested.

For me the difference is about does the person care about the answer - or do they just want to know.

I once worked with a woman who was unbelievably nosey, but if ever anyone called her out on it, her answer was always the same........
"I'm not being nosey, I just want to know"!

I have a friend who asks questions but doesn't listen to the reply because she's thinking or asking the next question. It's making me not want to be in her company. She asks questions about family that I wouldn't ask them myself. I'm sure she's the same with everyone.
I don't think she's nosey, I don't even think she's that interested.

Not everyone wants the nitty gritty. I have enough going on in my life, so I don't have to get my kicks from being nosey.

Yes marbles I agree - I think the line is right there. Between asking the polite stuff and asking the questions that would give tyou the answers you actually want. Let's be honest - no one is really interested in the niceties. We want the nitty gritty

Sometimes it's so frustrating to only be able to ask the polite questions when all you really want them to do is dish the dirt

And where do you draw the line between 'people watching' and 'staring'?

I like to 'people watch'

I agree it's when the questions become intrusive. General questions like holidays, family, etc. It's if you start asking about personal things like money and sex life that people might think it's nosey.

That said I've had great conversations about both those topics but they were with people I'd known a long time lol.

In my experience the line between curiosity and nosiness is usually to do with money. So if I ask you what the weather was like on your holiday or what hotel you stayed in that's curiosity but if I ask you how much you paid for the trip or how you managed to afford it that's nosiness.

I'm a student of human nature - that's my excuse!!