It takes 10 to 30 minutes, on average. It's not a big medical secret.
So a patient might need a top up? So therefore don't give one?
No, you can't predict how quickly a baby will make his appearance -- but again, that is no good reason not to give one surely? You don't know if it's going to take two hours more or six. Epidurals are still given in hospitals where pain management for women in labour is a priority despite the unpredictability of childbirth. It's a question of where your priority lies.
If you are going to insist that every woman in labour gives 'natural' birth a trial and to resist pain medication, then you will find all sorts of silly reasons not to co-operate with a patient's request for an epidural, many here on this thread. If that's what your hospital is about, then patients should be informed before it's too late to take their business elsewhere.
Epidurals were developed with labouring women in mind. The anesthesiology profession and the pharmaceutical industry are well aware of the circumstances of labour and how fast or slowly it can go. Yet they persisted in developing the epidural and in developing techniques associated with it, both for initial administration and in topping up, in monitoring of patients while it's in effect, and in researching the best window for the needle to be inserted -- they need to know how much a woman is dilated; yet another group of professionals is on here questioning the work of the anesthesiologists, making epidurals sound far more risky than they are, and putting up all sorts of really nonsensical objections to the process of administering them at the right time. It makes absolutely no sense unless there's an agenda of making women try birth without pain relief, a political agenda that actually disempowers individual women in the name of empowering them.
The risks I mentioned are small. And you are there to treat/ deal with the prolapsed cords and the crash sections etc., whether they happen after or without epidurals. You are a medical professional and that's your job. And so is pain relief when that relief comes with very, very small risks and great benefits.