I'm not entirely sure how long the fellowship is, but I would guess 2 yrs. You're probably not getting a lot of feedback because neuro-onc is one of the smaller subspecialties of neurology.
As the above poster stated, this subspecialty is in high demand right now. Many academic institutions are lucky to have just one neuro-oncologist. I have been told many times that it is different from the other neuro specialties in that you often become your patient's primary care doctor for all intents and purposes. Treating brain tumors is usually a fairly long process that is quite overwhelming to the patient. With the other neuro subspecialties you act more as a consultant.
Neuro-oncologists direct chemo regimens for their patients and typically have a favorite neuro radiation oncologist, neurosurgeon, and neuropathologist to assist with other aspects of the case. I think it is a very exciting field that obviously has a great deal of room to improve with advances in chemotherapy and rehabilitation. The three neuro-oncologists I have worked with all took a great deal of satisfaction from their jobs.