This is a straight forward question with complex implications for the development of an athlete. It requires a shift in thinking away from quantity to quality. It is not how much but how well the work is executed that ultimately counts toward performance improvement. Optimum load is the goal.
Stimulus Threshold is the term I like to use, or the current buzzword Minimum Effective Dose is also very descriptive of what is necessary. The concept is to find the optimum training dose that will stimulate an adaptive response but will allow the athlete to recover sufficiently to train as necessary in subsequent training sessions.
Along with this we need to recognize that training is cumulative, it accumulates day to day, week to week, month to month and year to year. Therefore, we do not have do it all now, it takes time and taking time will ensure development of a robust adaptable athlete who can handle the loads imposed by the competitive demands of their sport.