Studies such as these can take into account a huge number of differing variables and can isolate factors far more precisely than those which only last for a couple of years. This study knows every detail about the participants lives and is therefore in a much better position to make judgements than shorter similar studies.
The Harvard Grant Study has come to the conclusion that professional success in life comes from having done chores as a child. The study was conducted over 75 years and spans many different eras. Although lifestyles have changed, the conclusion that doing chores as a child aids in professional success makes common sense and practical sense regardless of the time taken to perform the study.
No, the length of the study should not invalidate the conclusion. While there are other factors to success, I can see where chores for kids builds a lot of important skills. It shows them that jobs need to be done, and that they are capable of doing the job. In business, you have to get the job done, and I'm sure that having done chores as a kids reinforces this mental outlook as an adult.
No, the length of the study does not make its conclusion invalid because it was meant to show people at various times in their lives and during various generations. There is no reason we should dismiss a study for taking long. In many ways, it makes the study even better.