A lot depends on your physical condition and the kind of car you will be driving. Anyone in good shape can drive a comfortable car at least 600-800 miles a day even without a second driver. If your car is comfortable and trustworthy then half the battle is won. Cars with vibrations or problems cause worn nerves on long trips. Stop when you feel tired no matter how long you have been behind the wheel. Deep breathing can keep you alert and refreshed. Never push yourself.