The number of hours you put in each day depends on how many drivers there are. If it’s just you, don’t do more than 6 or so – and even that is a lot to do day after day. However, if you have a companion or multiple, then you can safely drive more like 8 or 10 hours.
Unless you’re really trying to get somewhere, save your driving for the daytime. That way you get to enjoy the scenery through which you’re passing and get a good feel for the area.