Dad most likely will stay in the Nursing Home. When's a good time to bring this up with mom?

Dad is not improving (gaining strength) enough to make the transition back to living back home. Mom seems to be okay living alone but we haven't had the talk about "what if" dad can't come back. When and how should I approach the subject? I know with dad, I need to continue to give him goals to improve so he doesn't slip into depression on never getting out. But there will be a point when he realizes he's there to stay. I'm starting to feel overwhelmed as their caretaker with wanting to make them both comfortable.