Five or six hours is my break even point. I used to do Cleveland to Chicago or DC a lot, which are both around a ~5 hour drive. Flying, I figured on ~30 minutes to the airport, 45 minutes of waiting around at the airport, 1:15 flight, 15 minutes to get out of the airport, 15 minutes on the car shuttle, 10 minutes to get a car and get out, and ~40 minutes to get to my meetings.

Add in the potential for delays, particularly in the winter, and I almost always ended up driving. Not to mention that $0.54 a mile in reimbursement makes for some nice extra pocket change: at around 750 miles round trip, that means a $400 check.