The initial "pie" assessment usually has 30 questions and like ColinsMum said, it depends on how you do which questions you get. In our math intervention program (obviously not gifted kids) if a child scores 50% or below, we automatically bump them down a grade. We're shooting for getting them in the 60-75% range and then working up.
The pie is visible from the parent/teacher login. If the teacher doesn't want to help, ask her to create a teacher account for you and then move your DS to that class. It takes about 45 seconds to do it and it's easy! Then you can see what he's working on, how many problems he attempted, how long he worked, create worksheets, move him from class to class and also ask for quizzes or assessments. It's all pretty user friendly and clickable.
I like ALEKS because it actually doesn't make you do a ton of questions. If you get one wrong in the work mode, it will add 2-3 more to the practice pile. If you get the first one right, you can usually complete a topic in 3-4 questions depending on the level. In school, this would usually amount to 3-4 whole worksheets spread over a week! That's pretty darned quick IMHO!
By the way, I know I sound like an ad for ALEKS... Sorry about that! I have found for a school setting it creates the easiest differentiation method with a resistant teacher and a nonsupportive school so I like it!