Rule 1: It’s Not Thirteen Weeks, It’s Three
This was the hardest one for me to learn, and it’s almost always the hardest to get across to both students and their clients. University terms may be thirteen weeks long, but students are usually juggling five courses, and many have part-time jobs as well. That means they can only put eight hours a week into their project without sacrificing grades somewhere else. If you figure a full-time work week is 35 hours, that means students actually spend 8×13/35 = a bit less than three weeks working for you.
Take a look at the rest of his excellent post on Three Rules for Supervising Student Programming Projects for some other hints about what his most successful students had in common. It's interesting reading for mentors, teachers and students.