I often tell my students that practicing is like eating a sub sandwich.
Maybe not quite as tasty, but the principle is sound, and it goes like this: you wouldn't take a sub sandwich and shove the whole thing in your mouth at one time—at least you shouldn't—and neither should you cram all of your practice into one time slot.
Research shows that we remember first and last things very well, but not necessarily what's in between, whether it's a list of numbers or the songs on a CD. If this is true (and I can say from experience that it is), then we need to approach practice with this idea in mind. If we have one 60 minute practice session, we have 1 beginning and 1 ending; two 30 minute sessions = 2 beginnings and 2 endings; four 15 minute sessions = 4 beginnings and 4 endings. All are 60 minutes, but all are not equally effective. By segmenting our practice into smaller units of time but more frequently, we increase our chances of retaining and remembering what we've done, and given that our society has trained us to have shorter and shorter attention spans, this method of practice just makes sense.
Like that sub sandwich that is far more satisfying if we divide it up and have some now, some later, and some tomorrow, our practice can be more beneficial, more fun and we'll do more of it in the long run. Now, go have dinner!