Dot man, there really are no rules.
Everyone is different and unique. I have never felt a need to artificially push someone to hang out. So, I would say "when it doesn't feel forced." As you get to know her, maybe you find out that she likes doing something that you also enjoy doing. Then maybe the next time you do it, you invite her as well.
How long does it take to get to know what she likes? I dunno, a few weeks, a few months? Some people are more open than others.
Sadly, it's not an equation that can be solved. It's something that really has to be made up as you go along.