Q. When does the pre-Pesach avoidance of eating matzah begin?
A. Rabbi Moshe Feinstein discusses the issue in Ig’rot Moshe, Orach Chayyim 1:155.
The generally accepted rule is to begin asking Pesach questions 30 days before the festival. But to make things easier, some said “from Rosh Chodesh Nisan”, i.e. two weeks before.
The avoidance of eating matzah before the festival follows the lenient view.
Note that not eating matzah in advance (except for Erev Pesach) is a custom, not a strict law.