I can't think in miles since a couple places i've gone for work have involved significant 45-55 mph driving.

5 hours is my threshold. It also depends on airports and price. I'll drive from CLT-Greenville, NC b/c the flight is turbo prop and usually seems quite expensive, then i'll still have to rent a car. To places like BWI the flights are cheap and I can get by with just shuttles.

For personal travel it depends on whether it's just me, how long i'll be there, and if I need a car when I'm there. more than 1 person = driving will be significantly cheaper. More than a few days and i'm willing to drive farther. I have never rented a car to visit family so if someone can't pick me up when i'm there, then i'll probably just drive the whole way!