My humble opinion is that you should not wait at all. Supposing that you stop now, your immune system will need about six months to return to normal. But starting LDN immediately will help your immune system "recover" much faster. Be prepared to experience some old symptoms (nothing new though) until LDN modifies your immune system.
I may be wrong, but it sounds sensible. I stopped Rebif and started LDN a week later because I had some problems passing LDN through customs (burreocracy for ever!) but I would start immediately. There are also some people taking both Avonex and LDN without any problem.
If God created MS, Devil would create Rebif to treat it!