Someone posed the question “When do you finally decide your book is a flop?” I saw all sorts of answers, up to and including one who said, “NEVER. There’s no time that a book is ever a flop.”

Well, I know that person was trying to be encouraging, but there is such a thing as a book that flops. Even famous writers have had them.

There is no one-size-fits-all answer to the question! It depends on WHY the writer is starting to think that the book flopped:

If it isn't finished yet, and the writer has just become increasingly unhappy with it, only the writer can decide whether it’s time to scrap it and start over. There’s not much anyone else can do.

IF it’s already published but there’s hardly any sales, I would definitely say the novel itself is not what’s flopping. THE MARKETING IS.
The author, particularly if he or she is self published, may simply need to upgrade or begin an advertising campaign that will get traffic to the link.

If the person is wondering if it’s a flop because all the reviews are bad, that’s a little scarier...
If the majority of the reviews are one or two stars, it may be time to read exactly what people are saying about the story and take it to heart. Then the writer can decide about making changes to possibly reverse the negative trend. Of course, make sure there’s enough reviews to count! If the book is 100% one star, but there’s only a total of TWO reviews, it might just be two people who don’t like that author and they’re being spiteful. LOL. But if there’s a lot of reviews and they’re overwhelmingly negative, THAT might be the best indicator that a book is flopping. But even if that’s the case, and it’s looking like the novel's a bona fide clunker, that DOESN’T mean the author should give up. It may just be time to try again. Hope that was helpful!