Actually, the brain can make do, depending on the age and health of the person, as well as where the mass was
located, the brain is pretty good and doing work-arounds. It may not happen immediately, but it can happen in some.
There is no hard and fast rule - specifics are needed - like the size, type, location and what the tumor or cyst is.
Some tiny ones can be troublesome depending on location, while people can have large ones and not notice for a long time.