The Sisters would say that they need to do Read to Self and Writing daily. So, try to divide their time. Remind them that authors have to take breaks, and often come back re-inspired to make their stories better :). Or, also, that we as adults readers hardly ever get to finish a book in one setting–find a good stopping place and enjoy the remembering of what they read and wondering about what’s coming up next :).
Of course, you know your students best, so there may be an exception now and then, but these are good guidelines to stick with for the most part.