I agree with Joanne and Mac. It depends on the person as well as if they have any medical conditions that will make it hard for them to concentrate and focus. It also depends on the type of training they are engaged in, the length of the program, the intensity and frequency, their daily lifestyle, work schedule and their commitment to the program you have laid out for them. It’s different form person to person, so giving you a definite answer here without knowing whom you are referring to it will be very challenging.
When I meet with my clients for the first time I try to take in consideration their lifestyle and habits, so I can design a program for them that it will be easier for them to follow and commit to it. If you don’t give them goals that are impossible or very hard to reach, then it will be easier for them to follow it. Break down their program into smaller segments with attainable goals, so they can stay with it and don’t feel they are failing because it’s very hard for them to reach them. Then evaluate the results and make any changes if needed so they stay engaged and committed to the program (and still enjoy the whole experience).
I hope this helps.