We like 6 days minimum, we take 2 days to drive each way, but we don't do it so we arrive exhausted. We don't mind getting home exhausted but not to jeopardize our safety on the road of course.
Our best schedule is leave home at 9 am, drive until 9 or 10 pm, sleep over in Savannah area, then get up and hit the road again by 9, arrive at WDW by 3 for check-in. That gives us that first evening in a park, plus 6 full days. To come home, we like to leave around noon so we can get a good night's sleep. We'll drive around 10 hours, sleep, then get home around 2 or 3 the next day.
The best way to do this for us is Friday through Sunday. if it's NOT in the summer. If you are going to drive down 95 in the summer, do NOT do it on a Friday or Saturday or you'll sit for 5 hours from Washington DC to Richmond VA, sometimes in stopped traffic.