I heard it was twice in two years. Is that right? Where does that come from? If I have no reason to assume they became pasul do I still have to check them?
An individual's mezuzah has to be checked twice in seven years. A community's mezuzah has to be checked twice in fifty years. Yoreh Deah 291:1.
You have to check them even if you don't assume they are pasul. In fact, even if you check three mezuzos, you still have to check all of the others (Pischei Teshuvah 291:1).
The Aruch HaShulchan (291:1) says that in a place which is exposed to the elements and moisture or rot are common, one should check more often than twice in seven years and at least once a year. He notes that this was his personal practice.
As Rav Herchel Schachter notes at approximately the 26:30 mark in this shiur, Rav Shlomo Zalman Aurbach clarified the halacha of checking twice every seven years, that it is not the case nowadays.
The twice-in-seven-years requirement was stated when the situation was heat extremes. But nowadays, where we have air conditioning in the summer and steam in the winter, the halacha of checking would be like that of tefillin -- check it once initially, and then continue to assume that it is good.
Furthermore, see section II part C of this article / notes by R' Aryeh Lebowitz for a fuller discussion. I'll quote the relevant portion in full:
Rabbi Shlomo Zalman Auerbach is cited as having ruled that if a mezuza remains enclosed in a plastic, and is completely protected from the wall and from the elements, one can check it much less frequently (שלמה על תפלה ד:נב). Rav Auerbach is cited as having ruled that tefillin do not require any checking either because they are made in a way that they are completely protected from the outside elements.
So, to answer the first comment, it would perhaps encompass outdoor mezuzot.
Indeed, I've heard it said elsewhere that repeated checking will wear down the mezuzot, and make them become pasul sooner.