It depends on how much of a connection you have already. This could happen within the same day of chatting, or it could take longer.
There are some women that understandably need to get good vibes from a man they are talking to online, before they would be open to meeting up. Other women are much more spontaneous and even if they're unsure about the guy they've been talking to, they will agree to get to know him more in person.
If you've been having a back and forth conversation with a woman online and you feel it's time to meet her in person, you have to gauge her comfort level with you. If you find the conversation flows easily and she's been pretty active and timely in messaging you, then it may be the right time to ask her out.
When asking her out, keep it casual and low-key. You don't want her to smell desperation. If she says she wants to meet you, but she's busy then don't give up. Ask what day works for her. Or ask her out again the next time you talk.
If a woman has no intention of meeting you in person, she won't necessarily come right out and say it. Instead, she will make excuses. She may enjoy talking to you and the attention you're giving her, but she's not into you enough to want to meet up with you.
And that's okay, there are tons of connections that don't happen for one reason or another. She probably thinks you're cool, but there isn't enough of a spark. Don't waste your time. Move onto someone who is really excited about you, and looks forward to meeting you.