Level III generally teaches the Usui Master symbol as well as the attunement process. There is time to learn the attunement and practice it. Understand fully that all of this material is added onto the Reiki system from Hermetic Magick, Tibetan Tantra, and New Age spiritism. This is not to denigrate these practices or to say that these are not legitimate practices of their own, simply that they were never part of the Reiki system before 1980 and are not in the least bit necessary to practice Reiki. Some consider these a good addition to the original 4 Usui symbols.
Most instructors require that the student wait a set period of time between levels.
This can be a set time, or at their discretion. 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.