The doc will normally let you know when you can get it. I got mine around 6cm and it lasted til about 45min after i had my daughter.
I think u can only get it till ur 8-8 1/2!!! But u can get one as soon as u get to ur hospital but most times they like for u to be 4 cm before so usually they will offer demoral (not sure of the spelling) with ur iv! I tell my hubby he has one job.....get me to the hospital in time I want my meds!
I got mine at 8 cm dialated.
I was able to walk 30 minutes after i had my daughter.
Maybe get it at 6 cm...? Seems like that's when a lot of women get it.
I would have got it sooner but I progressed super quick.