Different hospitals have different policies regarding epidurals. You should check with your doctor to find out the specific policy of your hospital. Recent studies have shown that getting an epidural early in labour (before 5cm of dilation) does not increase your chances of needing a C-section, as previously believed. So, it is likely that you will be able to get an epidural as soon as you think you need one.
Likewise, administering an epidural late in labour does not alter the likelihood of a C-section, or other interventions. However, placing an epidural takes about ten minutes, and it can take another 20 minutes to reach maximum effect. If your doctor believes you will deliver within half an hour, she may recommend against it. Additionally, getting an epidural requires you to lie perfectly still for a couple of minutes. During the late stages of labour this can be difficult to accomplish.
I hope that helps, please post any questions you might have!