Anytime you are in labor
Every hospital has different policies so be sure to check with your healthcare provider and hospital to see what their policy is.
Generally you can get an epidural anytime after you have gone into labor. Once the hospital has accepted you as a patient in real active labor you can get one. Sometimes you might have to wait if the anesthesiologist is busy with another patient.
Some healthcare providers suggest waiting until you are 3-4cm along to reduce the risk of labor stalling and needed a c-section. That is controversial and the medical studies don't really agree with that theory.