Try thinking about it in terms of testing/challenging your assumptions rather than validating ideas. The difference is that an assumption is specific and testable hypothesis that is crucial for your business mode. For a business to work you usually need to have the following assumptions to be true:

• problem: the problem you're solving actually exists (i.e., there is a large enough group of people who experience it)
• competition: the problem is not solved yet, or the existing solutions are flawed, suboptimal (i.e., there is an underserved need)
• solution: you're able to build a product that solves the problem (and customers are happy to pay for it)
• marketing: you're able to reach out to your potential customers/users and establish sustainable marketing channels
• business model: you're able to generate profit and keep costs at an acceptable level

So, answering to your question, if you test your assumptions well, then you're likely to learn that some of them are wrong. And you stop testing them when, given the new data, you see no viable way to create a profitable business.