I got mine at 6/7 cm dilated.
When u can't stand the pain anymrore
My friend got hers before her water broke because she got induced but I think if you can handle the pain to just go with it until its too much but make sure you don't wait too long or else they won't he able to!
I got mine at 8cm dialated.
That was my "last shot", so i took it.
When you think you need it..get it. Just don't do it to early. I'd personally wait until at least 4/5 cm.
I got mine as soon as I got to the hospital because I has to stay calm for my son who was in the room