Most people, when they first start public speaking, find they are worried that they won't be able to fill up the allotted time. They will "dry up". Good preparation will help to overcome this problem but the more often you give the same lesson the longer it will get.
There is a good way of adjusting the length of a lesson and that it to use the formula shown above.
Divide the overall presentation into the "must" knows, the "should" knows and the "could" knows.
MUST KNOWS are the things that the audience/trainees must absolutely be told else there is no point in them being there. Make sure all of these points are covered.
SHOULD KNOWS, as the name suggests, are important things to be covered but can be left out if absolutely necessary.
COULD KNOWS are things that don't matter that much such as anecdotes and the like.
Always keep the above three things in mind when deciding on the length of the lecture/presentation.