You can stop when you no longer have inflammation in your rectum. Knowing when that day comes is kind of tricky because inflammation can be present without overtly nasty symptoms. It took me several years to get free of it completely. Not kidding.
One more point - most of us believe that you should continue using your meds even when in remission. This seems to reduce the risk of another flare. You can use a rectal mesalamine enema once a week forever and maybe that is the way you stay healthy. That seems like a reasonable plan to me.
My IBD has never been identified as Crohn's or UC; I treat it as if it was Crohn's. I haven't had any rectal inflammation in years so I no longer use my enemas. But I have them ready just in case.
People with UC absolutely have an issue with recurring disease in the rectum. I would advise you to pay close attention to this and keep that inflammation away. The risk of your disease spreading up the colon beyond the rectum is real, especially if you don't treat the rectal inflammation aggressively.