Usually 6-8 hours, but it really depends on other factors. I'd fly STL-Chicago (5 hr drive) if I was staying in downtown Chicago and didn't need a car because most hotels charge so much for parking and STL-CHI flights are usually pretty cheap. KC, Memphis and Indy are all about the same distance as Chicago from here and I don't think I'd ever fly to one of those because it won't be nearly as expensive to park my car, and unlike CHI, I'd probably need a car anyways. Not to mention the fact that unless I want to go in a FedEx box, I don't think there are nonstops from STL-MEM or IND, so a connection makes it even less worth the time.

On the other hand, I don't mind a road trip every once in awhile and might be willing to go up to 15 hours away driving. But 6-8 is my rule of thumb.