Well there is something that you should cheek into, when I was looking for something compeatly different on the internet I cam across this, it's called a walking epidural. Basically, it lessens the amount of narcotics that you put them into your system... but here is the website that you can read up to, though it does say that walking is most likely not something u'll want to be doing along with the hospital might not allow you to be up and walking the halls, but trips to and from the bathroom are "okay." Any who read up on it...
http://www.justmommies.com/pregnancy/labor-and-childbirth/walking-epidural
I think it depends on the dr, but somewhere between 5 and 8 you do get cut off. It also depends on how long your labor is taking. If you are taking forever to dialate then you have better odds of getting an epi later in labor. I think most women go in too early because they are afraid of not being able to get it and then labor stalls.
My nurse screwed up and they gave me the epidural at 10cm because she didn't check me first. It made pushing impossible because I couldn't feel a thing!
I think it is if your pass 5cm they will not give it to you, you have to remember all women is different, but for some women it take hours to dialate but they still get strong contractions so after being in pain for 5 hours and only diallating 3 cm some women ask for the epidural, thats a long time to be in pain and dialating slowly. So most women are able to go awhile with the pain it just depends on how fast you dialate, again, it could take hours sometimes to get to 10cm but those contractions keep on rolling in and getting stronger.
I can't speak for all doctors, but MOST doctors won't give it to you past a certain point in your labor. If you are really dialated and then decide you want it, many doctors won't do it then because it tends to slow down labor. When you are almost there, they don't want to do that. So, as soon as I am able to get it, I do. I don't feel the need for it at that time, but I get it so I don't miss the opportunity. BUT, you could always get a different anesthesia later on in the labor process though.