There is no instruction manual on this, but if Theophylline and Terbutaline (T&T) were not preventing your episodes, and now IVIG is doing so, then it makes sense to stop taking T&T after a few months of overlap.
Doctors usually prefer to make one change at a time -- in your case, add IVIG to your treatment -- because that way they can tell if something goes wrong or right what it was that probably caused it. Since nothing has gone wrong, they probably should proceed to remove one or both of these medications.
In my case, my doctors and I waited 5 months until ending my treatment with T&T, but I have heard of other patients who didn't wait as long. With the benefit of hindsight, of course, I wish I had stopped taking those terrible medications the next day after getting the first infusion of IVIG.