How do you define success/fail for an MVP?

So I have a history in B2B-Saas where it was quite easy to sell things face-to-face, test out value offerings, close the deal, and then go back to the office and crunch out a product or a variant of the product. Meaning did i sell this thing if yes = Good, sell to more! Did I not manage to sell= Bad pitch, value offering or target=Go back and work on it. But when releasing an MVP, we think we have a good idea and have conducted both interviews, surveys and worked with product wizzes. So the hard thing now is we're releasing an alpa in a few days, and planning to roll out the beta in a week or two. How do we know if it's any good? How do you setup an criterias to succeed or fail when you don't have any benchmarks? It's a freemium service, a web application that doesn't require registration. So essentially we're planning on checking bounce rates, time spent, retention and virality. But when compiling surveys and interview questions, what tangible metrics would you say are good criterias?