I think it really depends on the route (and number of hours driving you're willing to put in each day). For example, I went on a cross country trip from Bay Area (CA) to NY in four days drive time. But that's driving straight for over ten hours a day. And then again, there's the stereotype of CA driving rather fast... so we were probably doing on average about 75-80 the entire way.
Anyway, I'm about a day or two out from Kansas, so I think three (if Lois is driving) or four days(for just about anybody else) is reasonable to assume for the drive to Kansas from Metropolis.
