A private mezuza must be checked twice every 7 years while a public mezuza is inspected twice every 50 years. What is the significance of these shiurim and why are they expressed in such an unusual manner? [The phraseology almost suggests that if one inspects a private mezuza after the first two years, he needs not recheck it for another 5 years. This does not seem to be rational.]
The Chachomim decreed that the Mezuzos of the public should be examined only twice in fifty years, while private mezuzos need to be checked twice in seven years. R' Chaim Kaniefsky Shlita in his sefer on Meseches Mezuza brings a dispute concerning the reason for the different shiurim.
(a) Rashi explains the reason the mezuzos need to be examined, because perhaps it rotted or was stolen. However, the Chachomim did not inconvenience the public to check as frequently, because by doing so, it would cause everybody to try to "pass the buck" to someone else, and nobody would end up doing the job.
(b) The Tosefas Yeshanim argues on Rashi's reason for the infrequent checking of the public mezuzos, and says the reason was that the checkers were exposed to danger from the government officers.
(c) The Sefer Ha'eshkol offers an alternative explanation. He says that private mezuzos were more likely to be damaged by exposure to the elements than public mezuzos, becausethe large amount of people passing by would ensure that public mezuzos were watched carefully.
R' Chaim Kaniefsky Shlita further states that although the phrasing of the Gemora (Twice in seven years and twice in fifty years) seems to imply that even if one would check a private mezuza two consecutive years he would fulfill his obligation for the entire seven years, this is not so. It is clear from the Beraisa in Maseches Mezuza that the obligation is every twenty five years or every three and a half years. Our Gemora was only using a "Lashon Ketzarah", an abbreviated manner of speech (perhaps to avoid discussing half-years, when discussing the private Mezuzas).