Gradually, then suddenly.
And it applies to almost any situation in life. '
How did your relationship end?' 'Well, gradually, then suddenly'.
'How did your career nosedive?' 'Well, gradually, then suddenly'.
It captures the dull and all too easy routine people fall into and the accompanying indifference that makes you impervious to the inevitable, so that when it happens, you knew it was happening for a while, that the end came before you actually realised it.
It's not about craft this line. It's about having seen and experienced a lot.
Like a Kannadasan song. (He was a genius poet/lyricist for movies in south India.