Mezuzahs should be checked twice every seven years. In many cases mezuzah checkers find that the mezuzah scroll has been slowly baked to a crisp over a long period of time, following exposure to the elements.
The cause can be hard to determine. In one instance a mezuzah in a cheap, not waterproof case survives over the course of many years without suffering any damage, whereas another scroll in an ostensibly waterproof, solid metal mezuzah case is utterly destroyed.
One mezuzah checker got some insights into this enigma when he made house calls to two homes side-by-side. Both had an aluminum mezuzah case with a screw cap beneath the parchment on the bottom. Both had a lot of exposure to direct sunlight. And both mezuzah parchments, despite the protective case, were burnt to a crisp. His conclusion was that a lot of sunlight, even without moisture, with destroy a parchment in an aluminum mezuzah case.
He therefore extolled the virtues of the plain white plastic mezuzah case with a plug on the bottom. They will turn yellow and look shoddy after a year or so, but the cost to replace this type of mezuzah case (it definitely should be replaced if it doesn’t look nice) is minimal and the mitzvah of mezuzah remains intact.