Setting up something with a bitch then she says something like "I'll tell you when I'm free".

I've seen it a lot here and I've come to take it to mean

"If I have nothing better to do, I'll see you"

So what's the counter.

If you get such a reply, in person not over text, just cancel.

"I'll tell you if I'll be free"

You: let's cancel. You should definitely do your duties. When I'm free next time, I'll let you know.

Exit.

This shows you are not waiting up on her.

The next time you set up, she'll remember this and be less reluctant.