Dr. Stikle refers to this phenomenon somewhat humorously as 'the seven year itch.'

7 seconds sound clip from the The Seven Year Itch (1952) movie soundboard.

You can hear this line at 00:58:50 in the Blu-ray version of the movie.

Quote context

[...]

- And the termites of sin and depravity gnawing at his soul.

- Well, how's that strike you, Sherman?

- Dull, dull, dull.

- 'Chapter Six. The Brubaker-Stikle Theory of the Sporadic Infidelity Pattern in the Married Male, or the Seven Year Itch.'

- 'A thorough study of 18,000 marriages conducted by Dr. Stikle and myself leads us to believe that the urge curve in the middle-aged husband rises sharply during the seventh year of marriage.'

- Dr. Stikle refers to this phenomenon somewhat humorously as 'the seven year itch.'

- Far from humorous, this unfortunate urge, which strikes 84.6 percent of the married male population rises to an alarming 91.8 during the summer months.

- Changed my mind.

- 'Case History 'A' - Gustav Maraheim, the Mad Lover of Leipzig.

- 'A rather extreme, but by no means unusual, case of the seven year itch occurred in Leipzig, Germany, in the year 1912.'

[...]