I like the 80/20 rule. 80 percent review, 20 percent new passages. Great formula for practice because it gives the student a well rounded work out on memorization, technique, tone, repertoire, and provides a solid foundation upon which to build new skills, learn new pieces and advance. It seems counter intuitive, but to play what you already know helps you learn the new pieces faster. If you only have 15 minutes, review. If you have lots of time to practice, play all the songs you know in the book you are working in, this also provides an excellent warm up!