This is a personal decision, one that can only come about as a result of your own interest in the work. It would be easier, wouldn’t it, if we had assurance that once you hit, say, four hundred and thirty-two hours of revision, you’ve done all you can? That’s just not the case, though, so the decision is up to you.
If you’re still engaged in the piece and eager to strengthen it, then continue revising. Be aware of how you’re revising. If you’re tinkering with language, even though you know the plot needs a major overhaul, you might consider how you can better focus your revision efforts. If you’re taking it apart in a more genuine and productive way, forge ahead.
However, if you’re not still engaged with the story, perhaps it is time to put it away. Even so, this might not be goodbye forever. Down the line, you may find that your imagination is reignited and you’re logging more revision hours on that once-abandoned story.
This issue highlights an interesting conflict in the revision process. Some writers are eager to have their work fit into neat categories. It’s either in progress and it’s done. For some, this can be an essential part of their forward momentum. Still, you might consider what it means to have a story that is in progress, but that you’re not regularly pulling out to work on. Perhaps there’s another category here: dormant. Some stories can simply hibernate. Some stories need this. After the cold months pass, you can reconsider its category.
All that brings us to this: Yes, some stories do just fizzle out. Give the stories—and yourself—ample opportunity, but don’t continue to flail at a story that no longer has your interest. Some stories (and even some novels) are learning experiences. See what you can glean from the process and move forward.