The first thing to ask yourself is what it is you want the pupils to be learning from the introduction. Once that is clear, the short answer on length is that it depends on your audience. If the pupils are enthralled and fully engaged, then no one is going to think your lesson introduction is too long. But if pupils are fidgety and their attention wanes, then they are unlikely to be getting as much out of your introduction as you want.
That means you need to know your pupils. If their attention spans are short, then they are likely to make better progress with short sharp bursts of input from you punctuating their more active tasks. In some classes, teachers try interspersing physical activities into listening sessions. For many pupils, a few moments being kept, quite literally, on their toes, can help to delay the restlessness that can set in when pupils are left sitting on their bottoms.
When pupils are involved in more passive listening, the way they are sitting can make a difference too. If they are on the floor at the teacher's feet, discomfort can distract sooner than if they are seated at their tables. But sitting at tables can also sometimes mean several pupils are facing away from the teacher and their attention can drift.
If behaviour is good in your school, it may sometimes mask the fact that not all are listening. Watch out for those pupils who are quietly inattentive. Those pupils who are happy to let the lesson waft over them are unlikely to be learning.
Where you adopt techniques for breaking up the period of passive listening, make sure they do not become mechanical and uninvolving. Many pupils have a knack of spotting where an activity is being gone through out of fashion rather than purpose. The use of "talk partners" is popular in primaries - with teachers posing a question or point to think about and asking the pupils to discuss it for a few seconds with the child next to them. This can be effective, but the teacher and pupils can sometimes just be going through the motions
Selwyn Ward has been an inspector for 15 years, working in primary and secondary schools. The views expressed here are his own. To ask him a question, email him at firstname.lastname@example.org.