I have been on the same dose, 600 mg 3 times a day, for about three years. It may be decreasing in effectiveness, I am noticing more burning, tingling, and the feeling of things running across my feet lately. It does work for me in that if I skip a dose I have electrical zaps and more burning.
I will probably not increase this dose any time soon because I do not currently have side effects, and I would expect to at a larger dose. I have not gained weight on it, as many do, and with my muscle weakness i definitely want to avoid that. Most of my pain, I have CMT (hereditary neuropathy), comes from my legs, and I take oxycodone for that.

Supplements are not really that helpful in my case, except for maintaining general health- b12, fish oil, d3.