The heart rate zone that is chosen and the amount of time spent training in it will depend on many factors such as training time available, fitness level, training phase, strengths and weaknesses and the specific event the individual is training towards. It can be difficult to know how long to train in each zone but this will mostly be determined by the event for which the individual is training.
The average fitness enthusiast or amateur athlete generally only has a limited amount of time for training each week, unlike professional athletes who may have 30-40 hours a week for the next 12 months to prepare for a particular competition. Therefore they will need to make the most out of their limited training time by ensuring their training covers the essential phases. It is critical to ensure their program involves the building of a solid, basic fitness level as well as also training at the intensity at which their event or sport is performed.
Every event is different and places specific demands on certain heart rate zones. Training must reflect those same zones so that the individual can be fully prepared for the event.