Are muscle cramps (particularly in the legs) normal after epidural steroid injection? If so, how long will they last?

Normal. If it does not resolve in a week, call your doctor to see what options you may have regarding this increased pain. Although uncommon, it can happen and usually will go away on its own in a few days.
ESI muscle cramps. Not uncommon & can be secondary to mechanical and/or chemical irritation of the nerve roots along the injection site. This will usually settle down in a couple of days, but may take longer. If these symptoms are severe, persistent and if you have any new onset numbness or weakness you should contact the physician who performed the injection to see if further evaluation is indicated.