How long do you think a good course should be? The answer is "It depends". I don't believe in standards such as a good course should not be too long, too short, and so on. How can we reach a conclusion without doing our research and understanding the requirements? A good course should be as long or short as it should be. Not helping, am I? Let me try and explain.
The duration of a course is decided based on stakeholders' requirements, learner profile and requirements, training gaps, table of content, organizational objectives, and learning objectives (if I have missed anything important, please do share). The more focused the objectives, the easier it would be to allocate duration. If it covers everything, it is bound to be ridiculously long and not very effective. For these kind of situations, we can suggest that the course be split into focused sections or parts. For a particular project, we came across a situation where the learner was not be able to spend more that ten minutes at a stretch for learning. We designed the course such that the content was divided into learning nuggets of 5-10 minute duration. You may have also come across situations where the learner is able to devote only half an hour per day for training. Now, this does not mean that we cramp everything within that half hour.
Think of a really good movie you saw. What is not important is how long is the movie but how good it is. I have seen two hour-movies that have been extremely painful to sit through. On the other hand, I have also seen 3 hour movies that I have enjoyed. When the movie is good, you don't realize how much time has passed. Now, this does not mean that we make our courses really long. Remember, how irritated we get when an unnecessary song or scene is a part of a movie. This is what I meant by courses should be as long or short as they should be.