hardcoremore Unfortunately, it seems to be for the whole app life-span, as @Dimensionscape stated. If for some reason, you switch from a paid tier to a free tier, you are supposed to republish your app with the free SDK, which includes a splash screen. This is probably one of the rules I'm the most uncomfortable with, because it could potentially involve a lot of work (imagine you have many AIR legacy apps that, after some time, don't sell enough to make $25/year, you'll have to republish all of them with a splash screen). If you want to keep your apps splashscreen-free, you'll have to pay forever.
Ultimately, I prefer a system where at least AIR 33+ checks the developer's license on their servers every time the app is launched. If the license is still active, don't display the splash screen. And if it isn't paid any longer by the developer, display the splash screen. That way, we would not have to go through the tedious process of re-publishing our apps ourselves on all stores.