Cleaning for Pesach

Questions sent to Rabbi Aviner:

Question: If a person thoroughly cleaned a room (vacuumed, moved furniture etc.) a week or a few days before Pesach so they are certain that there is no chametz, do they have to perform "Bedikat Chametz"?
Answer: The custom is to check a room quickly if one has thoroughly cleaned it on the off chance he missed something.

Question: How far before Pesach does one NOT have to bring chametz into a room for it to be considered chametz-free?
Answer: Thirty days.