As initiates progress through the Outer Order a number of obstacles can get in their way, which can distract them from the Work and slow or stall their progress. Just as some people try to rush through the grades, others might become stuck in their current grade and end up with too much of that element.
The problem is particularly apparent in the grades of Practicus and Philosophus, which relate to the elements of Water and Fire respectively. These two elements are the extremes and these grades are not on the Middle Pillar, like Zelator and Theoricus, so they are intrinsically more imbalanced.
Dwelling too long in these grades, in particular, but also in any of the other grades, can create an extreme over-abundance and imbalance of that element in a person, to the point of it being potentially dangerous. The paths leading to these Sephiroth, and certain other parts of the Golden Dawn system, attempt to mitigate the intense imbalance of these grades, but ultimately the only way to truly balance Water is to bring in Fire, and then find the tongue of balance between them in Air and Portal.
This idea of balance, which is present throughout the entire Golden Dawn system, can also be applied to the pace of grade advancements. We need to be careful not to rush and not to be too slow, as both create problems, one in not allowing ourselves time to assimilate the energies and learn the lessons of the grade (and to affect real change in our lives), and the other in not giving ourselves the necessary push onto the next stage of our growth.
Personally I have found, working with the Golden Dawn and other traditions before it, that six months is usually the time when the fullness of the element is experienced, and that spending less time means the energy has not yet peaked, while spending more time means the negative side of the element, such as lethargy in the case of Earth, starts to become manifest.
This may not apply to everyone and it is largely dependent on the amount of work a person is doing in a particular grade, along with the amount of change needed in that part of an initiate's life. But I've found it to be a good guideline to follow, both personally and for those who ask me about recommended grade time.
This partially explains why the minimum times for the grades in the original order were so little. Initiates might spend as little as three or even one month in a grade. Things have changed somewhat since then, as there tends to be a lot more in the Outer Order nowadays than in the original Golden Dawn, but there is a time when an initiate, and whoever is in charge of them in their Temple, needs to ask what is holding their advancement back and try to address it before it becomes a bigger problem.