How much does your landing page matter when launching an app?

I’ve been thinking about how much time app founders should actually spend on their website before launch.
You can spend days trying to make the landing page perfect, but if you're still figuring out your audience and messaging, some of those decisions might change anyway.

For people building and launching apps, do you usually build the website first and polish it before launch, or get a simple version live and improve it as you learn from users?