When (given my circumstances) is it ok to tell a girl I have feelings for her?

So there’s a girl that I met online 3 days ago. We’ve been talking a lot since then and I’m unsure just yet but I think there might be something there.

I’ve made the mistake too often of asking a girl out too soon and scaring them off, so I don’t want to scare her at all.

What do I do? How long do I wait and how do I find out if she likes me back?