How long might this ic flare from and epidural steroid injection given 4 days ago last? how long does it take that medication to leave the body?

Depends.: You'll have to ask the doctor who gave you the injection how long it last because there are different types of meds. As far as your ic i would go back to basics and work on diet, no caffeine, try zyrtec daily, prelief with food, take your Elmiron (pentosan) and other prescribed meds if you're on any.

3 weeks: The steroid can last up to 3 weeks depending on formulation.

3-4 weeks: Speak with your pcp or gu doc to get advice on what to do.