There are no set guidelines for this since the practice is so variable. Different people assimilate the energy at different rates, as well as practice more or less than others. Someone that is good with handling energy and uses the system to self heal daily might integrate the energy faster than one who was not as familiar or did not use the energy regularly. It is important to know up front what the fees for the class are, as well as if it provides certification. These fees are based on observations of the prices practitioners charge for training.
Class size is important.
Classes that are too big often lack personal attention from the teacher. The class should be big enough to allow you to practice on a variety of people. Avoid people that promise outlandish things from an attunement. If they are promising fireworks, enlightenment or other things I would tend to avoid them. Attunements are generally a relaxing pleasant spiritual experience, but even this generalization lacks the infinite individual response to an attunement.