My DD6 took about 2 hours for the WISC, maybe a little more. She took a brief break and just stopped to eat a snack when she needed (without coming out of the office).

Her tester had orignally allowed for about 1 hour for the WJ-III, and she also went over 2 hours.

The more advanced academically they are, the longer it can take; but as mentioned above, it depends on many other items like the tester's style.