This should vary vastly depending on terrain.
In fact, due to that, I'd say its up to you to "stat out" the terrain before your party get to it.
Remember that there's a lot of Skill Challenge type stuff you can put out there: imagine your party hiking the Grand Canyon, rim to rim, in August.
You've got some kind of skill challenges to deal with cliffs ..
You've got some kind of skill challenges to deal with finding good routes ..
You've got some kind of skill challenges to cross rivers ..
You've got some kind of skill challenges to avoid random encounters ..
You've got some kind of skill challenges to represent the elevation changes in that heat ..
Compare and contrast to a two-day hike along a well-traveled road between two cities - you still might have skill challenges, but they'd be much more social-oriented.
Of course, you set the DC's to level-appropriate, and force a speed of movement that fits your other campaign goals.