When going through other education classes I have heard even shorter time spans for attention. I do find that given the self interest to a particular topic, I can attend to it more or less than the 15-18 minutes. I like the idea of changing things up frequently. Even if you have a 50-minute lecture to give, there is no reason why you cannot break it up wit a question/answer time, or a relevant demonstration. Sometimes just a stretch-break can help to wake students up and get them re-engaged in the learning. What are some ways you have used in your classes to help break up long periods of instruction?