Definition: The term the seven-year itch originally referred to an actual itch that tormented individuals for years.
The term took on new meaning with the release of Billy Wilder's 1955 movie, The Seven Year Itch, starring Marilyn Monroe and Tom Ewell.
"That supposed urge for infidelity after seven years of marriage is the meaning we now have for this phrase. It is now often extended to refer to an urge to move on from any situation, and not even limited to those of seven years' duration."
Source: The Phrase Finder
