Yeah, this is one of those areas that there isn’t quite rules for. I usually assume that when PCs are rolling through the wilderness, they don’t really “explore” the hexes they go through. they couldn’t possibly do so. If they don’t have an encounter, that doesn’t mean there aren’t lairs or monsters there, it just means they didn’t run into each other. After all, it’s a great big amount of space.
So, my thought is that it probably takes at least several days of dedicated exploration to ensure there are no monsters in a six-mile hex. That’s when I’d roll to determine its value (You know, after the characters have had a chance to actually chart the natural resources thereof.)
If the result was bad, I wouldn’t object to them moving on and repeating the process in the next square over. But I’d say that every six-mile hex, if you clear the whole thing, probably involves a fair amount of work. I mean, 32 square miles is a lot of space. If you imagine how long it would actually take to tromp through that much wilderness and make sure there isn’t a big old cabin full of goblins or an undead-haunted crypt in a glade somewhere…and make sure there isn’t a stream full of gold dust or whatever…having it take a week to “clear” a hex and establish its value is not at all excessive.
Obviously, it would be much faster out on a plain, where you could just ride your horses around, than in a hilly forest country like the Ozarks, where you could pass fifty yards from a nest of giant weasels and never see them.