This question is one I get frequently from new moms wanting to make the best decision possible about getting an epidural. This is an easy and tough question all at the same time, so I will share my general guidelines from what I have seen in the hospital.
For first births I recommend waiting as long as possible simply because first births can take awhile so if you get it too early you could end up lying down for hours and prolonging your labor. If you are looking for dilation, I would recommend waiting until you are at least a good 4 cm dilated before getting an epidural for the same reason. If you are able to wait until 6 or 7 cm this is ideal because you will be getting it right around transition, which is the hardest part of labor, but is generally quick compared to the rest.
What if you are having trouble dilating? These recommendations are thrown out the window. If you are having trouble dilating, it is likely that your body is struggling to relax to dilate. In this case, an epidural may be very beneficial in helping you to dilate. This is not a, "I haven't made any progress in an hour" recommendation. This is a, "I have been stuck at 5 cm for a few hours" recommendation, because the first part of labor can be long and slow and can take quite some time to get to 5 or 6 cm, especially if it is your first.
Trouble dilating could also indicate there is some kind of complication, and having an epidural may be a good idea in case a c-section becomes necessary.
If you are being induced, then it does not matter as much when you get an epidural because your labor is being done for you. The question you need to ask yourself is how long to I want to be stuck in bed?
For subsequent births, there is a little more ambiguity. Your first birth will likely give you a good indication of when you should get an epidural with your next.
As with everything else, pray about when the best time to get an epidural is because everyone is different. These are my recommendations based on years of experience. Often times you will know if and when you need it, even with your first.
How did you decide when to get your epidural?