Generally, I'll drive if it's less than about 4 hours. I live just under 4 hours away from Chicago and it's just about bearable. With that being said, back when I used to work in aviation and I visited offices at/near airports, I'd fly even if it was only a 3 hour drive just because there was no need to deal with rental cars, etc. I've done some 6 hour same-day turns driving and that's just way too much.