My neurologist used IVIG in the same manner when I was on Imuran. I have read that the IVIG treatments are only good for maybe four weeks. That appeared to apply in my case. I have not heard of a one-day booster treatment but I am currently going through and eight course treatment which I had never heard of. If you keep a journal go back and look at how long after your IVIG that you started getting week. And I would say that would be the timeframe that you would need another treatment.

To be honest I would go by my gut feeling of what I needed and tell that to the Dr. and see what he says. Only you know how you feel at certain periods of time and what helps and what doesn't.
Mike