There are some general guidelines but it definitely depends on the person and a lot of other varying factors as well. What may be considered a flop for one artist could easily be considered a success for another less popular artist, etc.
For me a flop is a song or album that does not have longevity on the charts and good overall sales. Peak doesn't really have much to do with it tbh. If a single peaks at 25, but goes platinum and stays on the charts for months does that make it a flop because of it's peak? No. Selena Gomez's singles come to mind, Naturally and Love You Like A Love Song didn't have that high of a peak but I wouldn't call them flops.
The definition of a flop varies depending on who you talking about. Standards for a mainstream pop gurl are definitely different than relatively unknown artists.
There are some general guidelines but it definitely depends on the person and a lot of other varying factors as well. What may be considered a flop for one artist could easily be considered a success for another less popular artist, etc.