Breaking a bad habit is not the work of a single day.
Tips
history
From 《周易·坤卦·文言》: 臣弑其君，子弑其父，非一朝一夕之故，其所由来者渐矣 - regicide and patricide are not the work of a single day; they build up gradually. The idiom has ever since emphasized that big things, good or bad, need time.
usage
Overwhelmingly used with a negation: 不是一朝一夕 / 非一朝一夕之功. The point is almost always that something CANNOT be done in a short time. Rare as a standalone positive phrase.
