Without for weeks or even months to see conclusive test results (95% statistical confidence).
In my last job we struggled with this. It was more useful, frankly, to go with our gut and qualitative information from sales inquiries we got (questions like “how did you learn about us” or “what made you think to contact us”). We also had a PR firm interview clients to understand how they perceived our brand.
For better or worse we used these qualitative bits of info to try and build useful landing pages. But a lot of it was unfortunately subjective and involved debate over how our marketing actually worked.
As your traffic scales, you can look for smaller and smaller effects. But the whole idea of testing variants can be applied pretty damn early.
Obvious answer: the more traffic, the better your results.