When do you know that you need to stop and move to another project?

Hey hunters! Reading about other success stories I noticed that a lot of people prioritize quick validation with very basic MVPs. If the MVPs don't get enough traction, they just move on with other ideas. This makes so much sense to do. But sometimes there is extra work needed to validate these ideas. A simple MVP landing page is not enough. Getting traction with these products is also hard. But when they take off, they fly high. Check Airbnb story. How do you know if it's worth building forward if you don't get any traction at first? Can you share your stories?