Will my radiation cystitis go away once I'm done with radiation therapy?

Yes. Radiation cystitis means irritation of bladder because of radiation. It is usually short lived and should resolve within a couple of weeks after radiation. The severity depends on the amount of radiation and the volume being radiated.
Usually. If the bladder was in the radiation field the lining can be irritated. Once treatment is over the lining will heal and cystitis goes away. However if high doses are needed directly on the bladder its possible that it becomes a recurring problem and more permanent. Even when temporary there are medications that can help until it improves.
? If you are having frequency and urgency, you need a urine analysis, and speak with your doctor. I treat a lot of prostate cancer and colorectal cacner and rarely see this.