From my experience, I disagree with the heuristic of money paid within the first month for knowing if you are really talking to users, because it doesn't apply especially for enterprise SaaS, where you likely to have a direct sales model, and as a co-founder with no sales experience at the beginning, as you continue to sell subscriptions, let's say the same functionality within a product, you will improve as a salesperson, closing deals earlier and by that heuristic, you know what your customers want.<p>Consistent user engagement with the product is a better metric to build such a heuristic, because user engagement is direct correlated to value IMO. That's why I am a fan of even charging $1 per month for a product, because it easier (relative to having given it away for free) to increase pricing based on the value you deliver to your customer, which can be measured through user engagement with the product.