Never give up - when to apply this?

I've read lots of entrepreneurs' advice, and most of them boil down one concept:

never give up.

This is easier to say than to do. My question is, when is the time to give up and move on? When is not being too stubborn? When should we persist and keep trying again and again until we succeed? How do we not fall into the state of being stubborn, or foolishly over-confident about an idea or startup?

Thanks.

I think it depends on how you define your goal. If the goal is too narrowly defined, you may give up because you reach a point at which you have categorically proven that your plan was fatally flawed. But in most cases, during the process of failing, you may uncover an alternative approach or an adjacent opportunity that has better chances of succeeding than your initial idea. If this is the case, then not giving up would mean rearming and going after this revised plan.

This argues for defining a goal that is lofty and compelling enough to be worth throwing all of your resourcefulness and creativity into pursuing, even if this means going back to the drawing board several times. Very often when a startup fails, it is not because the opportunity turned out not to be there or because the product could not be built, it is because the team (and their backers) ran out of ideas or patience or both and simply gave up.