Let's please leave our personal beliefs out of this and focus on the literary/historical/scientific aspect.
I recently came across a claim:
The number 40 in the bible is used to signify that it was a large amount of something, not precisely 40. So "40 years" would be "more than 15 years, but we're not exactly sure how many".
Is there any substance to this claim? I was unable to find any sources myself, but I'm very much intrigued, it sounds plausible and sheds an interesting light on a lot of the stories.
This isn't really a controversial topic as far as I'm aware. I'm an interested layman and not a scholar, but I've seen this fact presented many times in different books/articles and usually without much ado.
For example, 2 Samuel 5.4:
David was thirty years old when he began to reign, and he reigned forty years.
In my copy of the The New Oxford Annotated Bible, widely considered to be the best scholarly version out there at the moment, the following annotation is written for verses 5.4-5 (emphasis original):
A chronological note by an editor which may be approximately correct, though David could hardly have been thirty years old at this time. Forty years is a favorite Biblical expression meaning a reasonably long time. Israel and Judah are kept clearly separate in the writer's mind.
I would be careful in assuming that every instance of the number forty in the Bible is metaphorical in this way, just like how in English you can find the phrase "and for a hundred years X happened", meaning that something happened for a very long time, but sometimes it really did last for close to one hundred years (ex. hundred years' war).