It is very tough, that's why the burnout is so fast for managers especially, but players also.
This is more of an issue than replacing Flick while getting a better manager, which is almost impossible. But someone with burnout is done. So if this approaches, and it will sooner rather than later in our situation...
Flick is a winner himself. He'll land at a far richer club after this. The longest spell at Barcelona in decades for a manager was 4 seasons. And that was Pep in a unique era.
This will have to come from Flick. For as long as he wants to stay here, nobody should fire him. Assuming that he'll leave himself if he really gets into a very tough spot of prolonged failures.