have you spent much time in South Africa before? Personally I would recommend that a better way to see Kruger is to go on a small group tour. Not as free and flexible as self-driving, but far less risky. As far as time needed, really depends on personal taste. About a week is the period I would suggest, as lets you get into the swing of things, but not so long that you get thouroughly sick of just gawking at animals.