That's something you need to gauge with the person you're talking to. When it feels right to ask them to meet in real life then ask them. Don't rush in too quickly though and ask to meet in the first message you send! Get to know them a bit first, talk to them on instant messenger or by phone if you can and when you feel it's time then arrange a date. Remember to meet somewhere public and follow the common sense safety tips that most sites have for meeting up for the first time.
