IVIG effects usually start after a couple of days and last about 4 weeks when they are effective… (they haven't been for me, I had two series without any effect…).
That treatment is rather expensive as they are prepared from human blood, they have also to be repeated every 5 to 6 weeks to have a permanent effect, that's why, besides their possible medical side effects, they are not favoured by some doctors. Some countries prefer plasmapheresis (about the same cost)
reported to be more effective, (again, they haven't been for me…), the anti-bodies are temporarily removed instead of being counteracted.
Maurice.