There's no timetable for how long fish can live with ich. Many fish never show any serious issues, while others succumb quickly. Even if you battle it back and don't see signs, it's there and will come back up when you introduce a new fish. So you're going to be stuck with not being able to get new fish as long as there is ich in the system.
The one thing to consider is overall fish health and responsibility. At best there are little parasites biting into the fish and causing irritation and stress and probably gill damage. At worst, they die. We've all had to decide at some point what level of discomfort we're OK with vs. the difficulty of actually solving the problem.
It's not SUPER hard to fix. You just have to get the fish out and put them somewhere safe for a bit, and make sure you're feeding/monitoring the main tank so corals get what they need. It's not great, but then you're setting yourself up for success.