How do you know when is the right time for giving up on your idea?

It's so hard to make that call, especially when it's something you're passionate about. It's all about balancing your available resources against the chances of recouping your investment. You never want to get yourself into a position where you're forced to make that decision because you've gone in the hole; it's time to pivot or revisit your idea when you can still survive. If you're six months post public launch and marketing, and no customers, then yes I'd consider moving on to the next plan. (And if you didn't do it for this project, for the next one, I'd gather a handful of early testers/users who will go on the journey with you and who can tell you if you're solving their problem).

But don't lose heart! Most entrepreneurs don't hit it out of the park with every concept; keep your creative juices flowing and be open to inspiration for the next idea.