How much time should I allow for an iQ question?

That depends on your class and the questions you are asking. An iQ is similar to a class discussion where students raise their hands to give an answer. Instead of selecting only one student to answer, all students get the chance to see if they know the answer. This means that there will be some slower responders and the usual fast responders, so this needs to be taken into consideration. Remember that by clicking on 'Update responses' you can see exactly who hasn't yet entered an answer.