Hypothetically, you could start seeing results in as little as 9 months if people coordinated and actually took it seriously. Even without any sort of concerted effort, I wouldn't be surprised if the U.S. sees a drop in birth rates in 2026 as large numbers of people independently (i.e. without coordination) say "no thanks!"
Part of the point of birthstriking is that even if the effort ultimately doesn't work, you've still succeeded in protecting your descendants from whatever thing you are striking against. For example, I don't think we can stop climate change at this point. But even if it doesn't save the environment, there is value choosing to not create new victims.