Between 20 and 25 questions - but it has a lot of information about the prerequisite relationships between topics, and the questions are carefully selected by the system to home in on where he's at (it's very clever, I do recommend reading the Research behind ALEKS section!) so it gets more information than you'd think possible with so few questions. One or two careless slips don't seem to mess it up much - the underlying theory does take account of them - but a lot would. It's supposed to take about 20 mins, but it's common for younger children to take much longer.
