@ddljohn-- I'm a nurse and my husband is a surgeon. As far as I know, no, a spinal schwannoma (or neurofibroma) may remain at a constant size for a long time but the issue is that it's not predictable. So it may grow two millimeters per year for two years, stop growing completely for another three years and then suddenly grow ten millimeters in one year.
So the answer to your question is, it depends. Doctors have to keep an eye on it so the patient has to go in for regular MRIs to see how it's doing.
Now, if the patient is experiencing problems because of the schwannoma like pain or numbness, doctors may decide to operate even though it's benign. It all depends on the specific situation.
It's also difficult for doctors I think because spinal schwannomas are rare. There are not that many doctors who have experience with these kind of tumors.