It would be helpful if you could let us know which return trip to Rockburg you are talking about.  There are a number of times in the book where they go from some place to Rockburg and you could say that each of these is a "return trip."

I will guess that you are talking about the trip in Chapter 13.  In this chapter, they have gone out through "The Gap" where they have found the ostriches and such.

On the way back, they travel all of one day.  After that day they sleep at The Woodlands.  We find out that they get back to Rockburg early the next morning.  So it takes them a bit more than one day.