Hey woods - Any kind of baking apple - Jonathan, Macintosh.  Avoid the "eating" apples like red delicious - they get too mushy.  As for time, I'd say about twenty minutes or so, depending on the heat of the coals of course.  Rotate and flip them a lot.  You can squeeze them with your hand or poke them gently with a stick to see if they're getting soft - just be careful not to puncture the foil and lose all the "good stuff" sizzling away in there.  Enjoy!