when would you think it would be OK to stop listening to God about "multiplying"?
Never.
You simply trust in God's wisdom and follow His commands. If we did that, then among the billions being born there would be enough absolutely brilliant inventors, physicists and engineers who would make space exploration and extraterrestrial settlement possibile, and we would have not one but multitudinous Earths to settle. Unfortunately, if we keep slaughtering our Einsteins and Teslas in abortion, or simply choose not to have them (because consumerism and personal whims and ambitions are more important), then we shouldn't be surprised when we turn up trapped on this tiny planet forever. Sure, the Earth is humanity's cradle, but "how long can one live in the cradle?", as somebody wise said.
India gives me some hope - the most populated country on Earth, poor by most standards, but the level of Physics in their highschools beats ours easily, hands down. Why does India have so many brilliant physicists and such a high level of Physics at schools? Because they have hundreds of millions of kids to choose from. It applies to other areas as well.
The West, however, is hopeless. God is saying "trust me: have children and love them, and I will take care of the rest" ("seek ye first the kingdom and the righteousness of God" and all that), and we are saying "no, children are a burden and an expense - we will slaughter them or won't have them at all". :-/
foetuses
Ever heard of partial birth abortion, Jon? Foetuses? Those kids would have been alive and perfectly healthy if some "doctor" hadn't murdered them.