According to the Oxford Advanced Learner’s Dictionary, an adult is "a fully grown person who is legally responsible for their actions", and responsibility here is an important point. In addition to undergoing training, every adult learner will have a lot of other responsibilities: work, children, household chores, pets, cooking, and much more.
Keep this in mind when planning
the timelines of your course:
it is better to break it into fragments that will be easy to assimilate between other duties. Start each segment to help the learner switch to the course topic from other matters.
Imagine you are an adult learner who is simultaneously thinking about how to redo a report at work, when to buy your child winter shoes, and what to cook for dinner, and at that moment, you are asked to think about the course material — you will likely need time to concentrate.