I'm not an oncology nurse, but I think I'm safe in saying that the prognosis is so highly variable that it's hard to answer your questions. There are several different varieties of "brain cancer," each with a different expected prognosis, and every person's experience with cancer is different and depends on many variables.
My first job out of nursing school, many years ago, our wonderful nurse manager, who was a young woman (in her 30s) with two little kids at home, went from complaining of bad headaches and seeming crankier than usual to having a craniotomy at the big teaching medical center and being sent home to die in what seemed like just a few weeks or so. She had a particularly aggressive type of cancer (although I can't recall the name now), and v. quickly got to the point where she resembled a seriously demented older person (in terms of her cognitive function) and couldn't be left alone. However, because she was young and in good physical health generally, she lived a (comparatively) long time before she finally passed away.
It was quite heartrending for all of us who were close to her, and esp. (of course) for her family. I have never forgotten the experience. Again, I stress that every person's experience is different, and that doesn't mean your friend's illness will run the same course.
However, I would visit sooner rather than later, if I were you, esp. since this woman is older. I visited my friend at home a couple times, once while she was still able to recognize me and talk a little about work, etc. (although she "drifted in and out" during the visit) and once later on when she was past recognizing anyone. The thing that has really stayed with me, though, is that I was certainly not this woman's closest friend, and was the newest staff member on the unit (others there had worked with her much longer than me), but her husband was so grateful and appreciative that I had made the effort to visit -- he said that I was the only person from work who had visited her at all, and it had meant so much to her and to him. So often, people tend to avoid persons with serious or terminal illness because of their own discomfort.
Best wishes for your and your friend --