Originally Posted by Nardini

The reduced tax payable when you buy your house in Italy is subject only to the house being your "prima casa" in Italy and that you get your residenza granted by your comune. There is no time limit in reality, so long as you have your residenza and you will not be asked to pay the increased "stamp duty" on your house - so long as it is your one and only house in Italy. It does not matter in the slightest where else you live, or for how long.

David: With your application for residenza accepted by the comune, they will now send a local policemen (usually still called the vigili) to check that you really are living there. You might try to ask when they are coming, but the chances are that the answer will be "sometime in the future", without any other detail. If you are not there when they call, you will have to start the process all over again, so, my advice to you is to arrange to stay there until you have being granted your residenza - then go wherever else you want to be.