There's no maximum sprocket ratio on a gear hub. If you wanted for some bizarre reason, you could definitely have insanely high gear inches.

I don't personally know the exact minimum sprocket ratio on the SA-8 hubs, but they come with a 25 tooth cog and Sturmey Archer makes 30 tooth cranks to go with them. That's a much closer ratio than any other gear hub out there right now, but the SA-8 is designed differently too, with first gear being direct drive (and there being no reduction gears at all)

The problem with a gear hub should never be worrying how high your top gears can get, at the very least. It's more of a problem with whether or not your other gears will be appropriately spaced and low enough at the same time.