Ask HN: How long to commit to your startup?

It's very common that I hear, "most startups fail because the founders gave up too early". Surely, determination and tenacity goes a long way in creating a startup, but at what point do you decide to cut your losses and move on. I find that the more time I put into something the harder it is to leave it behind because there is always that lingering optimism that we are at the cusp of exiting the trough of sorrow. What do you think are good indicators for leaving one's current venture for other opportunities?