'3 days and 3 nights' idiom

[borota]

A Hebrew teacher I was interacting with some time ago suggested that '3 days and 3 nights' was an idiom. As such it didn't necessarily mean exactly 3 days and 3 nights. It could also be a little more or a little less than that. A possible English paraphrase: "for days on end".

Mathew 12:40 paraphrased would then read:
'For as Jonah was for days on end in the belly of a huge fish, so the Son of Man will be for days in the heart of the earth.'

Question: Can '3 days and 3 nights' be understood as an idiom? Anybody found instances where it was clear that '3 days and 3 nights' was and idiom?