I've had one epidural with no relief from it, how far apart do the epidural have be? what is considereda large herniation?

Generally 2-4 weeks: Typically epidural injections are scheduled between 2-4 weeks apart to get efficacy. If you are not receiving much relief after 2 injections you may choose to see a spine surgeon for other options. It is variable as to what is a large herniation, however i would go with what the radiologist writes on your MRI report, especially if he says it is large or rates it a grade 3 or higher herniation.