Over on TheRPGSite Noisms asks how many hexes in a hexcrawl map should you have something preplanned for?
I would shoot for 10% to 20% detailed hexes. In the dozen or so sandbox settings I wrote so far that what I averaged. Out of 512 hexes Blackmarsh had 76 detailed hexes. Most people have a dozen detailed ideas, two dozen more sketchy ideas, and for the rest that why God invented random tables for. A good set of random table is worth it's weight in gold as it is easier to explain why something is then make it out of nothing.
Remember it is meant to serve as a framework for you, the referee, to fill out in detail as the players traverse the landscape, and have adventures. Not as a fully fleshed out world detailed to the nth degree. So in the end it boils down to whatever you need to get at the proper area in your "Bag o' stuff" to construct the various encounters the players run into.