I don't think conversion is that important of a metric when it comes to determining product/market fit - high conversion indicates you are good at convincing your customers are good at solving their problem, which is more of an indicator of your marketing prowess than actual product quality or usefulness (though obviously these are not entirely independent variables).
Retention is the ultimate number - i.e., for people who we have convinced to use the product, do they actually find it valuable and worth the money.
As for how high your retention numbers should be? Ideally as high as possible, in reality the floor is going to be the lifetime value of the user (retention_time * revenue_per_time) vs. the cost of acquiring the user. As long as the user's lifetime value exceeds their acquisition cost, your economics at least (barely) work.
Retention is the ultimate number - i.e., for people who we have convinced to use the product, do they actually find it valuable and worth the money.
As for how high your retention numbers should be? Ideally as high as possible, in reality the floor is going to be the lifetime value of the user (retention_time * revenue_per_time) vs. the cost of acquiring the user. As long as the user's lifetime value exceeds their acquisition cost, your economics at least (barely) work.