It's very important to know when to call it quits. If you are at the point where it feels like there is nothing more than you can do other than wait for a lucky-break, give yourself a hard deadline for an exit and stick with it.

This is what scares me about people raising insanely large $10 million seed rounds. It's too much runway. Takes too long to fail.

It took me 4 years of working on 3 different startups, with 2 different set of cofounders, before I landed on the idea/business that finally became successful.

Two of those failed companies were part of YC. Thank god every investor turned us down. It allowed us to shut down and move on to the next idea way faster.

Edit: If you're a founder sitting on a giant bank account with a startup that you're losing faith in... remember you aren't obligated to spend all that money. Investors will respect you for returning the money and admitting it's time to try something else.