Since they have never taken the test before, I would guess that the benchmark would be set very low (though I don't know this for sure). So I would warn your kids that they should not blow off the easy questions in the beginning -- questions will get harder based on how many questions they answer correctly.

As others have said, it is a computer-based test. It should take as long as it takes -- some kids will take longer to finish (many times because they keep answering questions correctly).