Specifically, before you push to it. This would be useful to create these events in advance.
It can vary, and isn't specified by the documentation. I would not rely on an RTMP URI generated more than 10 or 15 minutes ago. Unfortunately, you will have to create that live video post right before you go live.
You could always set up your own RTMP proxy so that your live encoders have a consistent address. Then, you just have to update the Facebook URI when you're ready.