This is all dependant upon the individual and the intensity that the person is running. if they are running very fast then you will very quickly fatigue until you can go no further. this is because they body is unable to meet the demands of the exercise. We do a test in sport science where we increase the intensity each minute until the subject can go no further. We are able to measure the amount of oxygen that they use and carbon dioxide that they produce. With this we can obtain data to identify something called VO2 max, which is the maximum amount of oxygen that the body can use to produce energy. We can also get something called ventilatory threshold. When we go above our threshold our body is unable to cope with the demands and we begin to fatigue quicker and quicker the further above this threshold we go. this would often determine when our body won’t be able to go any further.