Waiting for immigration decision for almost 3months

I need an urgent reply.I was removed officially from Belgium National register due to not having an official address for over 3months and I have gone to the commune where I live to officially change to my new address and the commune told me that they will have to ask the permission of immigration office in Brussel before they can officially register me back again.My question now is that how long does it take before immigration will take decision and what will be outcome of the decision because I am working and I will be expecting my tax paper so because my new address is not officially register in the national register in Belgium.The commune said Immigration has the right to say either yes or no.and if immigration ask them to register me then I will be officially be register back to Belgium national register.