Well I did have one with ds1, labour took about 8 hours, I think the epidural slowed it right down.
it was too late with ds2 - I asked to be transferred (home birth) when I was about to hit transition, and he was born half an hour later. MW looked at me and said there's two answers to that, darlin'. Bob hope, and no hope. (she was Irish)
Third time I asked for one on arrival at the hospital, I was around 6-7cm dilated and was told, the baby is coming, very soon - I insisted, as I wasn't coping, and I had to say it about 10 times before she actually went to get someone. She just kept putting it off, just having to fill in this form, or do such and such first - I think she hoped I'd do without tbh. No idea why - I felt quite cross about that...I didn't get a medal for going without last time, and I knew how much it was going to hurt.
Anyway I did get it in the end, but only about 20-30 minutes before he was born. So it was starting to kick in but it still hurt.
I'd speak to your MWs beforehand and ask when is the best time to ask for one - and how to ensure you do get it. I really thought she was going to put it off till it was too late, with ds3.