You know, I have been asked by a girl who gave me her number completely randomly to watch movies and it ended up being just movies. That, and I'm referring to her not just asking him when he's available outright as the lukewarm part. Saying he doesn't know when he's free is usually a solid "not happening today," so just ask him what time's good for a night together. Her thought process currently involves "I write this, thus he should write this back" and then she waits and tortures herself.
But as you're saying, just do whatever. This is just conversation. I don't want to put pressure down on anyone.